The antihypertensive efficacy and safety of once-daily amlodipine (5-10 mg) were studied in patients with essential hypertension. The study also included an assessment of the effects of single doses of amlodipine on platelet aggregation. Ten patients received amlodipine (mean daily dose of 7 mg) for 12 weeks in an open chronic study preceded by a 4-week placebo run-in period. Amlodipine significantly reduced the mean dorsal supine (-31/-20 mm Hg), sitting (-34/-23 mm Hg), standing (-34/ -23 mm Hg), and postexercise (-30/-20 mm Hg) blood pressures (BPs) at the end of 12 weeks of treatment compared with the placebo run-in period (p < 0.005), with no significant change in heart rate. At the end of a 4-week placebo washout phase following the chronic study, nine of the patients received an acute single 10-mg dose of amlodipine. Exercise testing before and 6 h after dosing showed that an acute 10 mg dose of amlodipine reduced BP without modifying the physiologic response to dynamic exercise. Amlodipine significantly reduced the degree of platelet aggregation in these patients (p < 0.005) induced by either collagen or ADP. This study demonstrated that amlodipine once daily was an effective antihypertensive agent and significantly inhibited platelet aggregation.