1. Amlodipine is a novel calcium antagonist which, although pharmacologically similar to other dihydropyridine calcium antagonists, has a long plasma half-life, permitting steady state blood levels to be achieved with a once-daily dose regimen. 2. We have performed a study to examine the effects of this drug on the blood pressure of hypertensive patients over a 24 h period. After a placebo run-in, the drug was administered to 11 patients at a starting dose of 5 mg, and increased to 10 mg after 2 weeks of treatment if the cuff diastolic blood pressure response was unsatisfactory. Cuff measurements were made at entry, after 2 weeks treatment with placebo, after 2 weeks on amlodipine 5 mg once daily, and after a further 4 weeks on amlodipine 5 mg or 10 mg once daily. Intraarterial blood pressure recordings were made at the end of the placebo phase and at completion of the study. 3. Mean supine blood pressure measured sphygmomanometrically was 168/103 (n = 11) mm Hg at entry, 169/104 (n = 11) mm Hg at the end of the placebo phase, 153/95 (n = 11) mm Hg after 2 weeks of treatment and 146/92 (n = 11) mm Hg at the end of the study. Blood pressure curves plotted for each phase of the study revealed an effective 24 h duration of action. Mean daytime blood pressure was reduced from 165/103 to 147/89 mm Hg (P less than 0.05, n = 10), and mean night-time blood pressure was reduced from 137/79 to 121/69 mm Hg (P less than 0.05, n = 10).(ABSTRACT TRUNCATED AT 250 WORDS)