The degradation of bradykinin by angiotensin-converting-enzyme (ACE) activity in cultured human endothelial cells was studied by direct measurement of bradykinin and by its effect on the release of endothelium-derived relaxing factors. The half-life of exogenous bradykinin (10,000 pg/ml) was calculated from the decay of the bradykinin concentration as 46 +/- 2 min in cell monolayers, 133 +/- 15 min in conditioned medium, and 24 +/- 2 min in homogenates. Most of the bradykinin-degrading activity in cell monolayers could be inhibited in a concentration-dependent manner by the ACE inhibitors lisinopril, ramiprilat, and captopril. Bradykinin-degrading activity was released into the culture medium containing one-fourth of the bradykinin-degrading activity found in the presence of cell monolayers. In cell homogenates higher unspecific bradykinin-degrading activities were present. The functional consequence of bradykinin degradation was demonstrated by the potentiating effect of ramiprilat on the generation of endothelium-derived relaxing factors nitric oxide and prostacyclin from endothelial cells. The study supports the concept of increased vasodilatory effects of bradykinin during ACE inhibition.