The discovery by Dr. Sapolsky and his colleagues is “quite intriguing,” said Joanne P. Webster, director of parasitic diseases in the School of Public Health at Imperial College London.

“It’s a small study, but it makes sense,” Dr. Webster said. “The rodents’ fear of cats is such a strong innate reaction. Something that’s going to override it is going to have to be quite a focused, fine-scale response in the mind of the rat. The neuronal activity underlying sexual attraction is a good candidate.”

Dr. Webster was the first to report, in 2000, that Toxoplasma infection piqued rats’ interest in cat urine. She found that infected rats acted normally otherwise, and only their survival instincts seemed to suffer. She called their response “fatal feline attraction.”

The Stanford researchers tested the brain activity of 36 infected and uninfected rats exposed to either the smell of a cat or that of an estrous female rat. They focused on two neuronal circuits — one for fear and one for sexual attraction. The two neuronal pathways run essentially side by side through a region deep in the brain called the amygdala, involved in emotions and many behaviors. To detect the level of neural activity, they measured the activity of a protein that is expressed only when neurons fire.

Toxoplasma infection may not directly cause the neuronal sabotage, said Patrick House, a graduate student in Dr. Sapolsky’s lab who carried out the bulk of the research. “The parasite could trigger an inflammation or some other response that in turn affects the brain,” he said.

Toxoplasma infection in humans almost certainly would not affect human behavior as it does the rat’s, Mr. House says.

At least two billion people worldwide are infected by the protozoan, many from eating infected meat. Initial symptoms are mild flu, after which the parasite forms cysts that lodge in the brain. There they remain for decades and are thought to have little or no effect in adults, except in people with compromised immune systems.