Forty-one patients with mild to moderate essential hypertension (sitting diastolic blood pressure (DBP) 95-114 mmHg) were randomised in a double-blind fashion to treatment with either amlodipine 5-10 mg once daily (n = 21) or captopril 25-50 mg twice daily (n = 20) over a period of 8 weeks. Office BP, heart rate and side effects were assessed during the run-in period on placebo, and after 2, 4 and 8 weeks' treatment. Blood pressure and heart rate were measured at the same time at each visit, 12 hours after the last captopril dose and 24 hours after the last amlodipine dose. At the end of the 8 week study, the reduction in sitting DBP was significantly greater (P = 0.002) with amlodipine. Ambulatory BP recordings were performed over a 24-hour period, at baseline and at the end of the study. Both treatment regimes significantly reduced clinic BP without affecting heart rate. However, amlodipine reduced ambulatory systolic (SBP) and DBP almost every hour over the whole circadian cycle, whereas the antihypertensive effect of captopril was attenuated during the final 3 hours of each dosing interval. The incidence of headache and peripheral oedema was identical between the two regimens. Only one patient taking amlodipine withdrew due to ankle swelling. This study demonstrates that the once-daily administration of amlodipine has a more sustained antihypertensive effect than does captopril taken twice daily.