This study evaluated the influence of dimethyl sulfoxide (DMSO) daily supplementation on diminazene treatment of trypanosomosis. Four groups of Trypanosoma brucei brucei infected rats received 7.0 mg/kg diminazene aceturate on day 7 post infection. Three of the four groups received different doses of DMSO (0.5, 1.0 and 2.0 g/kg, respectively) in addition to diminazene treatment. The changes in hematological parameters and the weights of liver, spleen and heart caused by T. brucei infection were monitored and used to assess the efficacy of treatment. The prepatent period of infection was four days. Infection caused significant increases in WBC count, spleen and liver weights but it caused decreases in PCV, HB and RBC by day 7 post infection (PI7). By PI14, spleen weight and WBC counts were reduced from the PI7 level without treatment. Diminazene/DMSO combination did not reduce liver weight or increased hematological parameters more significantly than diminazene treatment alone. Increase in the dose of DMSO caused increases in liver weight. Diminazene/DMSO combination reduced spleen weight more significantly than diminazene treatment alone. Diminazene/DMSO combination delayed re-emergence of parasites beyond PT21 at which time parasites were detected in the blood of rats treated only with diminazene. The limited advantages of diminazene/DMSO combination over diminazene alone as well as the possible liver toxicity of DMSO at high doses would not make DMSO supplementation a viable addition to trypanosomosis chemotherapy.