Experiments were designed to study the effects of trandolaprilat on endothelium-dependent responses in isolated blood vessels. Rings of either femoral or left circumflex coronary arteries of the dog or thoracic aortas of normotensive rats were suspended in organ chambers for isometric tension recording. During contractions induced by prostaglandin F2 alpha, trandolaprilat did not cause direct endothelium-dependent or -independent relaxation. However, when given to preparations incubated with angiotensin I or bradykinin, the compound evoked significant endothelium-dependent relaxation. By contrast, trandolaprilat failed to cause any change in tension when given in the presence of acetylcholine (ACh). In rings of femoral arteries, trandolaprilat potentiated the endothelium-dependent relaxation evoked by bradykinin and adenosine diphosphate; it did not modify the endothelium-dependent relaxations induced by ACh, substance P, or thrombin. In rings of femoral arteries without endothelium, trandolaprilat augmented relaxation induced by adenosine diphosphate (ADP) but not by adenosine. In perfused coronary arteries with but not those without endothelium, trandolaprilat caused relaxation in the absence of exogenous bradykinin (or ADP). These experiments suggest that trandolaprilat does not directly release endothelium-derived relaxing factor from the endothelial cells, does not interfere with the ability of the endothelium to release endothelium-derived relaxing factor, augments the endothelium-dependent responses to bradykinin (given exogenously or produced locally) and angiotensin I by direct interaction with converting enzyme, and potentiates the relaxation induced by ADP by augmenting its direct effect on vascular smooth muscle.