In a double-blind, randomized parallel-group investigation, a new angiotensin-converting enzyme-inhibitor, spirapril, was compared with a calcium antagonist, nitrendipine, in 266 patients with mild to moderate hypertension (diastolic blood pressure 96-119 mmHg). The object was to reduce the diastolic blood pressure measured 24 hours after intake of medicine to less than or equal to 90 mmHg. After monotherapy for four weeks with either 20 mg nitrendipine once daily or 12 mg spirapril once daily, the dosages were doubled in the patients in whom the desired blood pressure had not been obtained. After treatment for eight weeks, 12.5 mg hydrochlorthiazide daily was employed as a supplement in patients who had not yet obtained satisfactory blood pressures. Both methods of treatment resulted a lower number of patients who responded and lesser decreases in blood pressure than anticipated. No differences were found in the decreases in blood pressure resulting from the two therapeutic methods. The effect of supplementary hydrochlorthiazide to spirapril treatment was as anticipated while the combination with nitrendipine only resulted in a marginally extra decrease in blood pressure. Nitrendipine resulted in significantly more side effects and more patients defected from the investigation on account of side effects in the nitrendipine group (27%) than in the spirapril group (7%). This investigation had documented the abilities of nitrendipine and spirapril to reduce blood pressure and the side effects associated with this but does not predict whether the preparations can be employed to prevent the complications of hypertension which constitute the indications for treatment. Supplementing nitrendipine therapy with hydrochlorthiazide is not recommended.