During exercise a marked increase in plasma potassium in healthy subjects has repeatedly been demonstrated. In patients on treatment with angiotensin-converting enzyme inhibitors this may be further augmented. Therefore, the aim of present study was to evaluate the effect of enalapril on exercise-induced hyperkalemia. This was done in patients with acute myocardial infarction randomized to double-blind treatment with enalapril 10-20 mg per day (n = 7) or placebo (n = 6) within 24 h of onset of chest pain, and the results were compared with data from healthy control subjects (n = 11). Baseline plasma potassium did not differ between the three groups; i.e. 4.2, 4.0, and 4.1 mmol/l, respectively. An incremental, symptom-limited, bicycle exercise test was done one month after the myocardial infarction, and blood samples were taken for determination of plasma potassium. The exercise-induced increase in plasma potassium was not higher in the enalapril group as compared to the placebo and control groups, and there was no difference between the enalapril and placebo group, the specific values being 0.6 vs. 0.6 and 0.7 mmol/l, respectively. No difference was observed in the slope (dK/dt) between the 3 groups. In conclusion, enalapril at a dosage of 10-20 mg per day does not provoke any augmentation of the increase in plasma potassium during exercise.