The angiotensin converting enzyme (ACE) inhibitors delapril and enalapril were compared in a double-blind, randomized, parallel study. The population included 142 patients with essential hypertension who were treated with delapril and 140 treated with enalapril. At the end of a 2 week initial placebo period, the diastolic blood pressure of these patients ranged from 95 to 115 mm Hg in the sitting position. After this placebo period, randomization took place. Patients in the delapril group received 15 mg of the drug twice daily. Treatment with enalapril was started with a daily dose of 10 mg. If the diastolic blood pressure did not fall to less than or equal to 90 mm Hg or at least by 15 mm Hg after 2 weeks of treatment, the doses of the ACE inhibitors were doubled. After another 2 weeks, 25 mg hydrochlorothiazide daily was added if the target level of diastolic blood pressure was not reached. Both drugs caused a similar decrease of systolic and diastolic blood pressure. The antihypertensive effect of both ACE inhibitors was increased by the addition of the diuretic. The frequency and severity of side effects were similar in the two groups of patients. It is concluded from this study that delapril and enalapril do not differ in antihypertensive efficacy nor in safety.