Abstract In a placebo-controlled, randomized, crossover, double-blind study of 17 normal volunteers, we examined the effects of captopril on the concentration of opioid peptides during bicycle exercise and on quality of life after a 2-week treatment period. Two exercise tests (progressive exercise and constant work rate exercise) were performed. Maximum oxygen uptake and blood lactata concentrations were measured in progressive exercise tests. The exercise intensities corresponding to a 1 2 lactate threshold, a lactate threshold, and a 4 mmol/L lactate concentration were determined. Constant work rate exercise at selected work loads for 20 minutes was carried out to measure the concentrations of opioid peptides and other hormones. Quality of life was assessed after the 2-week treatment period. Captopril treatment had no effect on the exercise response of blood pressure, heart rate, maximum VO 2, and maximum work loads. The plasma concentrations of lactate, epinephrine, norepinephrine, and aldosterone increased during exercise and captopril did not change them. β-Endorphin levels and plasma renin activity also increased during exercise, and the increases were greater with captopril treatment. Met-enkephalin and leu-enkephalin concentrations did not increase during exercise. According to responses in the quality of life questionnaires, administration of captopril improved the physiologic state more than the placebo did. These findings suggest that captopril may act on the central nervous system involving an increase in the β-endorphin level.