The effects of combined treatment with the calcium antagonist isradipine and the angiotensin-converting enzyme inhibitor captopril were investigated in a randomized, placebo-controlled parallel-group trial comprising 28 patients with essential hypertension. The average age was 50 years (range, 31 to 65 years). After all patients were given captopril 50 mg twice daily plus placebo for four weeks, they were randomly assigned into groups receiving in addition either placebo or isradipine 1.25 mg twice daily in increasing doses at four-week intervals. During Weeks 20 to 24, the captopril plus placebo group was given hydrochlorothiazide 12.5 mg per day. Blood pressure was measured in the morning, 12 hours after tablet intake. Supine blood pressure was reduced in the captopril plus isradipine group by -8/-6, -14/-9, -16/-8, and -11/-7 mm Hg compared with the placebo group. Changes in diastolic blood pressure were statistically significant at Week 8, whereas changes in systolic blood pressure were statistically significant at Weeks 12, 16, and 20. With the addition of hydrochlorothiazide (Weeks 20 to 24), only supine systolic blood pressure was significantly reduced. One patient was withdrawn from the trial due to a rash. The results indicate that combined treatment with a calcium antagonist and an angiotensin-converting enzyme inhibitor is effective in lowering blood pressure and that the combination is well tolerated during long-term therapy. The combination of captopril and isradipine was more effective than captopril given with a low dose of hydrochlorothiazide.