Casual as well as ambulatory 24-hour blood pressure (BP) and echocardiographic parameters were studied in 40 patients with untreated or insufficiently treated mild to moderate essential hypertension. Left ventricular (LV) hypertrophy was assessed before and after 24 weeks of therapy with either the converting enzyme inhibitor perindopril or the calcium antagonist nifedipine. The design was a double-blind parallel study with a placebo run-in period. Patients received a daily oral dosage of either 4 to 8 mg of perindopril or 40 to 80 mg of nifedipine in slow-release form. A diuretic (25 mg/day of hydrochlorothiazide) was added in nonresponders (greater than 90 mm Hg casual diastolic BP). Once-daily perindopril and twice-daily nifedipine comparably reduced both casual and ambulatory BP throughout 24 hours (p less than 0.01) without affecting 24-hour heart rate. Six subjects withdrew from the nifedipine group and 4 from the perindopril group. After 12 and 24 weeks of therapy, LV hypertrophy was significantly reduced by both agents. Before active treatment was begun, LV mass index was more closely correlated to 24-hour (p less than 0.001) than to casual BP. This correlation disappeared after treatment with both agents. The correlation between ambulatory systolic day-time BP and LV mass was only still present (r = 0.54; p less than 0.05) after 24 weeks of treatment with nifedipine. It is concluded that regression of LV hypertrophy during converting enzyme inhibition or calcium antagonism may be partly independent of dosage and magnitude of 24-hour BP decrease.