Fosenopril sodium, a prodrug, is converted to its active diacid during and after intestinal absorption. Its excretion is equally divided between hepatic and renal routes. This placebo-controlled, randomized, double-blind, parallel group study evaluated the efficacy and safety of fosenopril compared to propranolol, at rest and during exercise, on blood pressure, plasma potassium, plasma renin activity, and plasma aldosterone. Exercise testing utilized bicycle ergometry, and individual subjects underwent an identical exercise protocol on placebo and on active treatment. The fosenopril group comprised nine subjects who were matched to nine subjects on propranolol. Blood pressure fell significantly and equally at rest (fosenopril--157/103 to 141/95 mmHg, p less than 0.005; propranolol--159/100 to 149/90 mmHg p less than 0.005) and during exercise in both groups. Plasma potassium fell significantly at rest (4.25 to 3.98 mmol/l, p less than 0.05) and during exercise (5.18 to 4.87 mmol/l, p less than 0.05) on fosenopril, but rose in subjects on propranolol during exercise (4.99 to 5.44 mmol/l, p less than 0.01). Plasma renin activity rose on fosenopril and fell on propranolol. Plasma aldosterone was uninfluenced by either drug. Fosenopril was well tolerated and its antihypertensive profile is similar to that of beta blockers and other ACE inhibitors.