Male mice (NMRI strain) of 3 and 5 g were inoculated i. p. with 8 x 10 6 and 9 x 10 4 metatrypomastigotes/g harvested from a 12-day-old LIT culture of Try panosoma rangeli of the "Dog-82" strain.At regular intervals after inoculation, the animals were sacrificed and portions of heart, liver, spleen, lung, thigh, kidney, stomach, intestine, brain, sternum, and vertebral column were embedded in paraffin, sectioned, and stained with haematoxylin-eosin and Giemsa colopho¬ nium.Pathology was encountered in the first five tissues cited above.The sub cutaneous, periosteal, interstitial, and peribronchial connective tissues, and later the muscle cells of the heart, were heavily parasitized by amastigotes and trypo¬ mastigotes.The possible reasons for the decrease in tissue parasitosis at the same time that the parasitemia is reaching its peak, and for the low level of in flammation in the parasitized tissues, are discussed.The observations of other workers, as well as the results described here, indicate that certain strains of T. rangeli under certain conditions may well cause pathological alterations in mammals.