The antihypertensive effects of lisinopril 20 mg once daily and slow-release nifedipine 20 mg twice daily were compared in a double-blind, parallel group, 10-week study involving 274 patients with mild to moderate hypertension. During the first 6 weeks of treatment, slow-release nifedipine and lisinopril produced similar reductions in lying and standing blood pressure (BP), except for lying systolic BP (SBP) which was reduced to a greater extent by lisinopril. After 6 weeks of double-blind treatment, hydrochlorothiazide 25 mg once daily was added if BP remained uncontrolled (lying DBP greater than or equal to 95 mmHg); a significantly greater proportion of patients in the nifedipine group than in the lisinopril group required additional diuretic treatment (29% versus 14%, respectively; P = 0.005). Moreover, after a further 4 weeks of treatment BP was adequately controlled (lying DBP less than 95 mmHg) in significantly more lisinopril-treated patients than in the nifedipine group (91.4% versus 78.3%, respectively; P = 0.006). Lisinopril was better tolerated than slow-release nifedipine. The frequency of drug-related events was significantly lower (threefold) for lisinopril than for nifedipine (P = 0.001) and the number of withdrawals from treatment with nifedipine was more than three times that in the lisinopril treatment group (P = 0.009). Lisinopril appears to provide an effective once-daily antihypertensive treatment which is at least as effective as, and better tolerated than, slow-release nifedipine.