Histochemical studies of myocardial biopsies from chronic chagasic patients at different evolutive stages showed a pattern primarily characterized by a marked increment in tissue enzymes such as mono-amine oxidase and lysosomal acid phosphatase. This cellular damage can be reflected by changes in certain serum enzymes associated with myocardial metabolism, specially in the coronary sinus, where the blood metabolized by the heart is drained. However, little is known about the possible changes in blood enzyme activity during chronic Chagas disease. In this investigation, the activity of the following enzymes glutamic-oxaloacetic transaminase (GOT), glutamic-pyruvic transaminase (GPT), alkaline phosphatase (ALP), acid maltase (AM), lactate dehydrogenase (LDH), alpha-hydroxybutyric dehydrogenase (alpha-HBDH or LDH1) and creatine phosphokinase (CPK) was measured in blood serum of the superior cava vein (SCV), coronary sinus (CS) and pulmonary (PA) and femoral (FA) arteries of 45 chronic chagasic patients, ages between 20 and 55 yr, at different evolutive stages (groups IA, IB, II and III). The results demonstrate that the average activity of the enzymes studied in chagasic patients, except LDH and CPK, are significantly altered (p < 0.05) in the majority of the arterial and venous blood samples. The finding of released GOT, GPT, ALP, acid maltase and alpha-HBDH in groups IA and IB is an indication of early myocardial damage in chronic chagasic patients without clinical evidence of cardiac disease. In conclusion, it is suggested that the possible evolutive pattern for myocardial damage could be established by the increment in coronary sinus blood of the enzymes GOT, acid maltase and alpha-HBDH