In Trypanosoma evansi infections changes in the haemogram are commonly observed, and the enzyme adenosine deaminase (ADA) plays an important role in the production and differentiation of blood cells. Thus, the aim of this study was to evaluate the activity of ADA in serum, erythrocytes and lymphocytes of rats infected with T. evansi compared to non-infected rats. Thirty adult rats were used, divided into 3 uniform groups. The animals in groups A and B were infected intraperitoneally with 2 x 10⁶ trypomastigotes/rat. Rodents from group C (control group), were not-infected. Blood collection was performed on days 4 and 20 post-infection (p.i.) in order to obtain acute and chronic infection stages of disease. The blood was used to assess the activity of ADA. In the blood, reduced haematocrit and increased lymphocytes were correlated with ADA activity in erythrocytes and lymphocytes. We observed reduction of ADA activity in serum and erythrocytes in rats infected with T. evansi compared to non-infected rats (P < 0.05). ADA activity in lymphocytes was decreased after 4 days, when the parasitaemia was high and increased after 20 days, when the number of circulating parasites was low. In conclusion, our results showed that the ADA activity was altered in serum, lymphocytes and erythrocytes of rats, concomitantly with haematological parameters, in experimental infection by T. evansi.