The converting enzyme inhibitor CGS 14824A was evaluated in 15 healthy male volunteers. First, the efficacy of a single 5 or 10 mg oral dose in antagonizing the pressor response to exogenous angiotensin I was tested in 2 subjects. Blood pressure and heart rate were monitored continuously through an intra-arterial catheter. CGS 14824A 5 mg reduced the response to angiotensin I within 75 min to 50%, and 10 mg within 1 h to less than 25%, and for a period of more than 4 h. Subsequently, plasma renin and converting enzyme activity, plasma angiotensin I, angiotensin II and aldosterone were measured serially before and up to 72 h following oral administration of 2, 5, 10 or 20 mg CGS 14824A to groups of 5 volunteers. Plasma converting enzyme activity fell to well below 10% of baseline within 1 h after administration of 5 mg or more CGS 14824A. Within 2 h following 2 mg p.o., a similarly low level was reached. Twenty four hours following the 20 mg dose, plasma converting enzyme activity was still below 10%. As expected, plasma renin activity and angiotensin I rose while angiotensin II and aldosterone fell following the 2 mg dose. This pattern of effects was enhanced by increasing the dose. Nonetheless, 24 h after the 20 mg dose, plasma angiotensin II and aldosterone had returned to their baseline levels. No side-effects occurred. Thus, in normal volunteers, CGS 14824A was an effective, potent and long acting converting enzyme inhibitor.