Pivalopril is a new orally active angiotensin converting enzyme (ACE) inhibitor. Pivalopril is a new compound with a hindered sulfur group that has been compared to Captopril for oral angiotensin-converting enzyme (ACE) inhibition in rats and dogs and antihypertensive activity in rats. In separate groups of conscious normotensive rats, Pivalopril (0.03-1.0 mg/kg, orally [p.o.]) produces a dose-related antagonism of angiotensin I (AngI)-induced pressor effects. The ED50 for Pivalopril and Captopril is 0.1 mg/kg. In conscious normotensive dogs, Pivalopril (incremental doses of 0.01-1.0 mg/kg, p.o.) produces a dose-related antagonism of AngI pressor effects. The ED50 is 0.17 mg/kg for Pivalopril and 0.06 mg/kg for Captopril. At equieffective doses the two compounds have similar durations of action. In sodium-deficient, conscious spontaneously hypertensive rats (SHR), Pivalopril (1-100 mg/kg, p.o.) produces a dose-related reduction in mean arterial pressure. The potency and duration are similar to those of Captopril. In the sodium-replete SHR, 5 days of oral dosing with Pivalopril (100 mg/kg per day) decreases mean arterial pressure more effectively than Captopril (100 mg/kg per day). It is concluded that Pivalopril is a potent, orally effective ACE inhibitor and antihypertensive agent.