As the production of oocysts by T. gondii in the cat seems to be correlated with the cystic form, once it does not occur when vegetative forms are given to cats, (17), brains of mice and rats with T. gondii chronic infection were orally administered to 32 cats. With TC1 strain none of the eight cats inoculated excreted Toxoplasma-type oocysts. With the 3 other strains the results were the following: "pombo" strain - 25%; "AS-28" strain - 37,5% and "sonia" strain - 37,5%. These results were interpreted partially as a consequence of the scarcity of cysts in the brains adminstered to cats and partially due to the existence in natural conditions of T. gondii strain which may be oocystogenic, paucioocystogenic and anoocystogenic. Diverselly from the results obtained when cats are fed with vegetative forms (17), none of the inoculated animals died on account of toxoplasmosis; microscopically the lesions with toxoplasms were very few and the number of negative RSF was relativelly high. A few evolutive forms of T. gondii were also studied in the gut epithelium of a cat without Isospora and, in the same manner as the oocystes, they are smaller than those of I. felis and I. rivolta. As cross immunity seems not to occur between the species of Isospora (I. felis and I. rivolta) it is possible that the same happens between the last ones and T. gondii. From a total of 185 cats examined only one (0,54%) excreted (in natural conditions) oocysts atributable to T. gondii. The role of the cat in the epidemiology of Toxoplasmosis is discussed emphasizing the fact that the discovery of the sexual cycle was decisive for the classification of the parasite, but was probably not so valuable for the epidemiological studies.