An acute form of trypanosomiasis (course of disease 4 to 8 days) was produced in mice experimentally infected with a strain of T. vivax. The features of the disease were: a parasitemia which appeared to increase exponentially from 23.8 to 243.6 times 10(6) trypanosomes per ml within the last 24 hours of the disease; anaemia was not severe in the disease in mice; the basic histological lesion was generalized fibrin thrombus formation in the blood vessels of the heart, lung, spleen and brain; trypanosome enboli were present in the brain, spleen and liver of infected mice. The disease in mice could prove a useful model in studies of the pathogenesis of acute trypanosomiasis and also in studies in the mechanism of generalized intravascular coagulation.