We conducted a double-blind, crossover study comparing the antihypertensive effects of isradipine versus captopril in patients with essential hypertension. Seventeen patients (8 men, 9 women; 6 whites, 11 blacks) completed both phases of the study, which consisted of two 5-week treatment periods separated by 2 weeks of placebo treatment. Each drug was randomly allocated to half the patients as the first drug and half as the second drug they received. Ambulatory blood pressure (BP) monitoring was carried out on the day before treatment and the last day of each active treatment. Both drugs were effective and well tolerated but isradipine was more effective overall than captopril in lowering BP (9.4% vs 3.9%, respectively; P < 0.02). Black patients had significantly higher BP at baseline than white patients; furthermore, black patients responded better to isradipine than to captopril. White patients had a smaller decrease in blood pressure with both drugs than did black patients, but white patients still attained a lower diastolic BP with captopril than did black patients (88 +/- 2 mm Hg vs 96 +/- 9 mm Hg; P < 0.01). There was no correlation between the pretreatment plasma levels of pressor hormones (plasma renin activity, catecholamines, and arginine vasopressin) and the magnitude of BP response to either drug, but the decrease in BP in response to captopril correlated significantly with the increase in plasma renin activity during treatment (r = -.84; P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)