WASHINGTON — The fact that alcohol dissipates from the bloodstream over time does not by itself give the police the right to draw blood without a warrant in drunken-driving investigations, the Supreme Court ruled on Wednesday.

The case arose from the arrest of Tyler G. McNeely, who was pulled over for speeding on a Missouri highway and, the State Supreme Court said, exhibited “the telltale signs of intoxication — bloodshot eyes, slurred speech and the smell of alcohol on his breath.” He performed poorly on a field sobriety test and was arrested.

Mr. McNeely refused to take a breath test and, after being taken to a hospital, to consent to a blood test. A blood test was performed anyway, about 25 minutes after he was pulled over, and it showed a blood alcohol level of 0.15 percent, almost twice the legal limit.

The state court suppressed the evidence, saying there had been no “exigent circumstances” that excused the failure to obtain a warrant. “Warrantless intrusions of the body are not to be undertaken lightly,” the court said in an unsigned opinion.