Toxoplasma gondii oocysts are highly infective to intermediate hosts including humans, pigs, and mice, but are considered less infective for cats, the definitive host. To determine infectivity of T. gondii oocysts for cats, 20 2- to 3-mo-old T. gondii-free cats in groups of 4 were fed graded doses of oocysts estimated to have 1, 10, 100, 1,000, or 10,000 mouse infective oocysts of the VEG strain of T. gondii. Feces of cats were examined for at least 35 days after feeding oocysts. All cats were killed, necropsied, their sera were tested for T. gondii antibodies, and tissues were bioassayed in mice. Three of the 4 cats fed 10,000 oocysts, 3 of the 4 cats fed 1,000 oocysts, and 2 of the 4 cats each fed 100 oocysts shed 7.3-162 million T. gondii oocysts in their feces, with a prepatent period of 18-44 days. Based on bioassay and antibody production, all 4 cats fed 10,000 oocysts, 3 of 4 cats fed 1,000 oocysts, 2 of 4 cats fed 100 oocysts, and 0 of 8 cats fed 1 or 10 oocysts acquired T. gondii infection. Antibodies to T. gondii were detected by the modified agglutination test in all 9 bioassay-proven T. gondii-infected cats and in none of the 11 cats without demonstrable T. gondii. In a series of other experiments, the age of the cat at the time of oocyst feeding and the administration of corticosteroids were found to have no influence on the prepatent periods after ingestion of oocysts. A review of published and unpublished data indicated that the minimum prepatent period to shedding of oocysts after the ingestion of oocysts by cats is 18 days.