Mountain hares and domestic rabbits infected experimentally with Toxoplasma gondii were compared in respect of pathological changes and distribution of organisms in the tissues. Seven hares and nine rabbits were each inoculated orally with 50 oocysts of the T. gondii Tg-SweF1 isolate and killed after one week to avoid adverse clinical effects. The only clinical sign observed was respiratory distress in a single hare. At necropsy, gross lesions, which occurred only in the hares, were found in the mesenteric lymph nodes and occasionally the liver. Histologically, the hares showed extensive necrotic areas in the small intestine, mesenteric lymph nodes and liver, and less prominent foci of necrosis in various other organs. Immunohistochemically, T. gondii was identified in all organs examined, large numbers of organisms being associated with the lesions. These findings were analogous to those seen previously in naturally infected hares. In the rabbit, the pathological changes consisted mainly of accumulations of mononuclear cells in the liver and heart, and T. gondii was found only in the heart and skeletal muscle of two rabbits. Thus, the hares developed severe acute toxoplasmosis, but the rabbits showed few organisms and no major tissue damage one week after inoculation.