Rats are considered to be 1 of the most resistant hosts for Toxoplasma gondii infection, but relative infectivity of T. gondii for rats is not known. Therefore, infectivity and pathogenicity of oocysts of the VEG strain of T. gondii were studied in Sprague Dawley weaned rats (approximately 130 g). Groups of 5 rats were each inoculated orally with 1 to 1 million infective oocysts. Three of the 5 rats fed 1 million oocysts died of acute toxoplasmosis between 6 and 9 days after ingesting oocysts; all other rats survived. Tissue cysts were found in brains of all rats fed > or = 10 oocysts and in 3 of 6 rats fed 1 oocyst. The average number of tissue cysts in brains of rats was 300, 180, 528, 600, 396, 1,200, and 2,650 in rats fed 1, 10, 100, 1,000, 10,000, 100,000 or 1 million oocysts, respectively. Microscopic lesions were seen in brains of all T. gondii-infected rats and the frequency of lesions was usually proportional to the dose. Antibodies (> or = 1:512) to T. gondii were detected in sera of all infected rats 29 days after ingestion of oocysts by the modified agglutination test, the commercially available latex agglutination test, and the indirect hemagglutination test.