1. The antihypertensive efficacy of once-daily amlodipine was studied in a group of 30 patients with mild to moderate hypertension in a double-blind, placebo controlled parallel group study. The dose range of amlodipine was 2.5-10 mg daily titrated at 2 weekly intervals for a total treatment period of 8 weeks. 2. Amlodipine produced a significant reduction in blood pressure compared with placebo, the mean difference between baseline and 8 weeks (corrected for placebo effect) being 16/12 mm Hg supine, 14/4 mm Hg standing. 3. Blood pressure returned to baseline values during a terminal 4 week washout period on placebo. 4. There were no significant effects on heart rate. 5. Two patients experienced slight ankle oedema while receiving amlodipine 10 mg daily but the active drug was otherwise well tolerated. 6. Plasma concentration of amlodipine, sampled 24 h after the preceding dose, increased as the dose titration sequence was followed, averaging 2.5 ng ml-1 on 2.5 mg, 4.9 ng ml-1 on 5 mg and 10.5 ng ml-1 on 10 mg.