The antihypertensive effects of moexipril, a new angiotensin converting enzyme inhibitor, and verapamil-SR as add-on therapy to hydrochlorothiazide (HCTZ) were investigated in patients with moderate to severe (stages II and III) essential hypertension. Of 147 patients treated for 4 weeks with 25 mg/day HCTZ, 108 whose sitting diastolic blood pressure (SDBP) was 100 to 114 mm Hg inclusive were randomized to 7.5 mg/day moexipril (56 patients) and 180 mg/day verapamil-SR (52 patients) in addition to 25 mg/day HCTZ. If after 4 weeks of treatment the SDBP was > or = 90 mm Hg, the dose of moexipril was increased to 15 mg/day and that of verapamil-SR increased to 240 mg/day; the patients were followed for an additional 8 weeks. Blood chemistries, plasma renin activity, and plasma aldosterone were done during the study. Both moexipril and verapamil-SR were safe and well tolerated and decreased the sitting and standing blood pressure similarly (P < .001). This study demonstrated that the addition of moexipril and verapamil-SR to low dose HCTZ is effective for the treatment of moderate to severe hypertension.