Ten patients (mean age, 46 years) with mild to moderate hypertension received 5 mg of amlodipine daily for 12 weeks. The amlodipine dose was increased to 10 mg daily in 4 patients whose blood pressure remained > or = 90 mmHg during the first 8 weeks. After 8 and 12 weeks of treatment, mean blood pressures in the supine, sitting, and standing positions and after exercise were reduced significantly. Heart rate did not change significantly from before to after treatment. Six hours after amlodipine administration, however, slight but significant increases in heart rate were noted at rest and after exercise. Platelet aggregation induced by adenosine diphosphate or collagen was significantly reduced 6 hours after amlodipine. One patient reported headache after the 10-mg dose of amlodipine. No other side effects were noted. It is concluded that 10 mg of amlodipine once daily is safe and effective in the treatment of mild to moderate hypertension.