The authors studied transaminase leveis (G-OT and G-PT) in the serum of 25 monkeys (Cebus apella) before and after variable periods of infections with S. mansoni and after treatment with aminonitrotiazol (Ambilhar). The average leveis in non infected monkeys were 28.3 units (Reitman & Frankel method) to G-PT and 24,7 units to G-OT with standard deviation of 11,2 and 11,3 respectively. The extreme values were 10 and 60 to G-OT and 15 and 57 to G-PT in this qroup. In the infected group only one monkey had an abnormal levei of transaminase (70 units to G-PT) which appeared 81 days after inoculation with cercariae of S mansoni. No change was found in the transaminase leveis of treated animais.