We assessed blood pressure (BP) and heart rate (HR) responses in a double-blind, randomized study comparing cilazapril, a long-acting, nonsulfhydryl-group converting enzyme inhibitor, with placebo in 18 patients with mild to moderate (sitting diastolic BP, 95 to 114 mm Hg) essential hypertension. The BP and HR parameters were evaluated at rest (casual, 24 hours after administration), during treadmill exercise testing (Bruce protocol), and with 24-hour noninvasive ambulatory BP monitoring. These assessments were made after a 4-week drug washout period and after 8 to 12 weeks of therapy. After 8 weeks of therapy with cilazapril (mean dose 3.6 +/- 0.9 mg/day), casual BP decreased 19/11 mm Hg (p less than 0.01), whereas placebo lowered BP by 4/5 mm Hg (difference not significant) compared with the baseline period. The casual HR was modestly (7 beats/min) but significantly (p less than 0.05) lowered by cilazapril monotherapy. Exercise BP was reduced by cilazapril (reduction at peak HR, 23/11 +/- 10/5 mm Hg; p less than 0.05), and exercise HR was unchanged. Compared with baseline, the duration of exercise was improved with cilazapril but not with placebo (1.0 minute vs -0.2 minute; p less than 0.05). Twenty-four-hour mean, awake, and sleep BPs were reduced with cilazapril with the most impressive reduction occurring during the awake period (19/12 mm Hg; p less than 0.01). These data demonstrate that cilazapril lowers casual, exercise, and ambulatory BP with a modest but significant improvement in exercise time. Thus cilazapril may be particularly effective in the physically active hypertensive patient.