The development of sporozoites to tachyzoites and bradyzoites was studied in mice after feeding 1-7.5 x 10(7) Toxoplasma gondii oocysts. Within 2 hr after inoculation (HAI), sporozoites had excysted and penetrated the small intestinal epithelium. At 2 HAI, most sporozoites were in surface epithelial cells and in the lamina propria of the ileum, and by 8 HAI, T. gondii was also seen in mesenteric lymph nodes. At 12 HAI, sporozoites had divided into 2 tachyzoites in the lamina propria of the small intestine. By 48 HAI, there was a profuse growth of tachyzoites in the intestine and mesenteric lymph nodes of mice fed 7.5 x 10(7) oocysts. Parasites had disseminated via the blood and lymph to other organs by 4 days after inoculation (DAI). Toxoplasma gondii was first isolated from peripheral blood at 4 HAI. Tissue cysts were visible histologically in the brain at 8 DAI. By using immunohistochemical staining with anti-bradyzoite-specific (BAG-5 antigen) serum, BAG-5-positive organisms were first seen at 5 DAI in the intestine and at 8 DAI in the brain. Using the bioassay in cats, bradyzoites were first detected in mouse tissues between 6 and 7 DAI, and they were found in intestines before they were found in the brain. Cats fed murine tissues containing bradyzoites shed oocysts in their feces with a short (< 10 days) prepatent period, whereas cats fed tissues containing tachyzoites did not shed oocysts within 3 wk. Using a pepsin-digestion procedure and mouse bioassay, bradyzoites were first detected in brain tissue at 7 DAI and in many organs of mice at 51 and 151 DAI. Individual bradyzoites, small and large tissue cysts, and tachyzoites were seen in the brains of mice at 87 and 236 DAI.