One hundred sixty patients with mild to moderate essential hypertension entered this double-blind parallel group comparison of amlodipine, verapamil, and placebo. Sixteen centers from the United Kingdom and Norway participated in this multicenter study. Administration of amlodipine (2.5 mg with adjustment to 5.0 mg and to 10.0 mg o.d.) or verapamil (80 mg b.i.d. titrating to 160 mg b.i.d.) for 8 weeks to patients with mild to moderate hypertension was associated with clinically important and statistically significant decreases in both supine and standing blood pressures. After 8 weeks of double-blind therapy, the mean (amlodipine-placebo) difference in the falls from baseline in blood pressures measured 24 h postdose were -11.9/-7.0 mm Hg (supine) and -11.4/-6.3 mm Hg (standing). The corresponding figures were -7.7/-4.6 mm Hg and 8.3/-4.0 mm Hg for verapamil (12 h postdose). Mean reductions from baseline of the amlodipine group exceeded those of the verapamil group by 2-4 mm Hg for all pressure measurements, and this difference was statistically significant for diastolic blood pressures. Responder rates were 72, 48, and 33% for amlodipine, verapamil, and placebo, respectively. Mean doses at 8 weeks were 9.1 mg (amlodipine) and 296 mg (verapamil). There were no significant changes in pulse rate. Both active drugs maintained their antihypertensive effect through the day (0-12 h postdose) with amlodipine showing a sustained 24-h effect. Twenty-two patients in the amlodipine, 19 in the verapamil, and 14 in the placebo groups had side effects possibly related to treatment.(ABSTRACT TRUNCATED AT 250 WORDS)