One-hundred and five patients with hypertension received nitrendipine (10-40 mg/day) or hydralazine (50-200 mg/day) in a double-blind randomized design. Nitrendipine decreased supine blood pressure 15/10 mm Hg, and hydralazine decreased it 11/11 mm Hg. Standing blood pressure was decreased 15/12 mm Hg by nitrendipine and 12/11 mm Hg by hydralazine. Supine and standing heart rate rose significantly after both drugs. Blood pressure variation through one dosing interval increased 42% when hydralazine was given but was not altered by nitrendipine. Side-effects from the two drugs were similar in patients who completed the study, but six patients discontinued participation due to side-effects of hydralazine, while only one discontinued due to nitrendipine side-effects. Ten patients required propranolol for hydralazine side-effects, and only three required it for nitrendipine side-effects. Nitrendipine and hydralazine are equally effective as antihypertensive monotherapy in patients with mild to moderate hypertension for periods up to 7 weeks, but nitrendipine appears to be better tolerated.