SAN FRANCISCO, California (CNN) -- A man snuck into the grizzly bear exhibit at the San Francisco Zoo Saturday but was rescued immediately afterward without incident, authorities said.

A zoo visitor alerted the facility's staff about the 27-year-old man's presence inside the enclosure, said spokeswoman Gwendolyn Tornatore.

Staffers fired a warning shot to keep the two bears away from the man, before securing them inside their dens, Tornatore said in a statement.

The man, who was not identified, was conscious but unresponsive when he was rescued. He did not show any apparent signs of injury, Tornatore said.

It was not clear how the man got inside the enclosure, or what prompted him. Watch barriers man had to get past »

The man has been charged with misdemeanor trespassing and harassing an animal in captivity, said CNN affiliate KRON-TV. He was taken to San Francisco General Hospital for an evaluation.

The zoo made international headlines on Christmas Day in 2007 when a Siberian tiger apparently scaled a 12-and-a-half-foot wall surrounding its enclosure, killing a 17-year-old man and wounding two of his friends.

Following the incident, the zoo overhauled its safety procedures. Construction crews raised the walls of the enclosure to help keep the big cats in.

The one-acre Hearst Grizzly Gulch at the zoo is home to its two female grizzly bears, Kachina and Kiona. It is equipped with a 20,000-gallon pool, herb garden, and a waterfall, according to the zoo's Web site.

CNN's Janet DiGiacomo contributed to this report.

All About San Francisco Zoo