Defense lawyers will be permitted to show that Trayvon Martin had marijuana in his system when he and George Zimmerman had their fateful encounter in a gated community in Florida last year, the judge in Zimmerman's murder trial ruled Monday.

Judge Debra Nelson denied a motion by prosecutors to keep toxicology results showing THC in Martin's system from the jury, paving the way for Zimmerman's lawyers to argue the drug may have influenced the teen's behavior. Defense attorney Don West noted that in Zimmerman’s statement to the non-emergency 911 dispatcher that it appeared the person he was observing in the Sanford, Fla., community was “on drugs.”

Late Monday, Zimmerman defense spokesman Shawn Vincent said the defense could rest by the end of the day Tuesday or early Wednesday morning. The prosecution would then have rebuttal witnesses, with closing statements by Thursday or Friday. Vincent stressed this scenario depended on everything going as expected.

Nelson's ruling on the toxicology results came after the jury was dismissed following a day of conflicting testimony about whose voice could be heard screaming on a subsequent 911 tape. A parade of Zimmerman's friends said they recognized the neighborhood watch volunteer's voice, but Martin's father, who police said initially told them he did not recognize the voice, said he came to realize it was his son after hearing the tape some 20 times.

The testimony, in week three of Zimmerman's murder trial, came after the mothers of both Zimmerman and Martin testified Friday it was their son doing the screaming in the Feb. 26, 2012, 911 call made to Sanford, Fla., police. The issue is critical because Zimmerman, a 29-year-old neighborhood watch volunteer, says he shot the 17-year-old in self-defense as he was being beaten.

Sanford Police Det. Christopher Serino and Police Officer Doris Singleton both recalled playing the 911 tape for Tracy Martin days after his son was killed, and both said the elder Martin did not recognize his son's voice on the tape. But later, Tracy Martin, curiously called to the stand by lead defense attorney Mark O'Mara, took the stand to say that he did recognize his son's voice on the tape later, after hearing it several more times when played at the Sanford Mayor Jeff Triplett's office.

"After listening to the tape for, maybe 20 times, I said I knew that it was Trayvon's voice," the elder Martin said under questioning by O'Mara. Martin called his son his "best friend in life," and said the boy's death turned his world "upside down."

Earlier, close friends and former co-workers of Zimmerman testified that they recognized Zimmerman's voice on the the 911 tape that captured his confrontation with Martin. Sondra Osterman, wife of Zimmerman’s best friend Mark Osterman, told jurors that the shrieking voice on the tape was “definitely” that of Zimmerman.

“Yes, definitely, it’s Georgie,” Osterman said. “I hear it, I hear him screaming.”

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Osterman told jurors she has known Zimmerman since 2006, when they worked at a mortgage company. Osterman’s husband has written a book about the fatal shooting — "Defending our Friend: The Most Hated Man in America" — and Sondra Osterman told jurors that would not affect her testimony in the case.

“I wouldn’t lie for him or for anybody,” Sondra Osterman said of Zimmerman; she said she was unsure how many copies her husband's book has sold.

Osterman testified she did not think Zimmerman's use of an expletive on the 911 call indicated any ill-will or hatred on his behalf.

"I don't think he was angry," she told jurors.

Mark Osterman, a federal air marshal, told jurors he discussed gun safety with Zimmerman and took him to a gun range. Osterman testified that Zimmerman was “very safe all the time” with his Kel Tec 9-mm. handgun, which does not have an external safety and is difficult to fire accidentally.

“It’s a reliable firearm,” said Mark Osterman, who recommended to Zimmerman that he keep a round chambered in the weapon.

Mark Osterman also testified that he heard Zimmerman screaming on the 911 call.

“It sounded like George,” he told jurors.

During cross-examination by prosecutor Bernie de la Rionda, Osterman said he was unsure how many copies his book has sold, but confirmed that all proceeds were destined for Zimmerman.

Geri Russo, a friend who previously worked with Zimmerman, testified that the yelling heard on the 911 tape is Zimmerman.

“I recognize his voice, I’ve heard him speak many times,” Russo told jurors. “I have no doubt in my mind that’s his voice.”

Leanne Benjamin, who owns a real estate company and formerly worked with Zimmerman, also identified Zimmerman as the screaming voice on the 911 call.

“I know his voice,” Benjamin told jurors. “I know what his voice sounds like when he gets excited or loud.”

Benjamin and her husband, John Donnelly, contributed $2,500 to Zimmerman's defense fund and Donnelly purchased suits for Zimmerman to wear during his trial, she testified.

Donnelly, who said he thought of Zimmerman as a son, later testified he found it “distressing” to listen to the 911 tape, which he recently heard twice online and again on Monday in court. Donnelly said he also recognized Zimmerman’s voice as the shrieking individual on the 911 tape, becoming the seventh witness since the defense began its case Friday to testify that the former neighborhood watch volunteer is screaming for help on the 911 call.

“There is absolutely no doubt in my mind that is George Zimmerman and I wish to God I didn’t have the ability to understand that,” Donnelly said.

Donnelly testified he also donated an additional $500 to Zimmerman for his website and purchased $1,700 in suits for the former neighborhood watch volunteer.

Further bolstering the defense contention that Martin was getting the better of the fight when Zimmerman shot him was Adam Pollock, owner of a kickboxing gym where Zimmerman trained prior to the incident. Pollock said Zimmerman was "grossly obese," and not atheltic at all.

"He came to the gym to lose weight and get in shape," Pollock said.

When O'Mara asked him to rate Zimmerman's athletic ability on a scale of one to 10 when he began training, Pollock replied, "Point-five."

Also on Monday, prosecutors asked Judge Debra Nelson to prevent Zimmerman's defense attorneys from showing jurors a computer-animated depiction of his confrontation with Martin. A motion made public Monday requests that the animation not be mentioned or played at Zimmerman's trial.

Prosecutors — who believe the animation would only confuse jurors — say the animation doesn't depict a murder weapon and only approximates positions based on witness accounts.

Zimmerman's attorneys called their first two witnesses Friday after prosecutors rested their case and Nelson denied a request for acquittal. Some of the witnesses called by the defense could be the same neighbors and investigators called by prosecutors. Defense attorney Mark O'Mara told reporters Friday he expects his case to last a few days and may recall some of the prosecution's witnesses.

The mothers of both Zimmerman and Martin testified on Friday that it was their own son's voice calling for help on a taped 911 call minutes before Zimmerman fatally shot the teen on Feb. 26, 2012. The screams are considered to be crucial pieces of evidence because they could determine who was the aggressor in the confrontation. An FBI expert testified earlier in the week that a person familiar with a voice is in the best position to identify it.

Zimmerman, 29, has pleaded not guilty to second-degree murder, claiming self-defense. It remains unclear whether Zimmerman will testify, although jurors already have heard his account through videotaped police interviews played in court.

Zimmerman faces life in prison if convicted. The state argued during its opening statement that the neighborhood watchman profiled and followed Martin in his truck and called a police dispatch number before he and the teen got into a fight.

Fox News' Joshua Rhett Miller, Serafin Gomez and The Associated Press contributed to this report.