The antihypertensive effect of 20 mg of nitrendipine (BAY e 5009) was compared with that of 40 mg, administered orally once daily to 12 patients with essential hypertension (WHO I-II). The study was designed as a double-blind, within-patient comparison. The present results indicate that nitrendipine has a significant antihypertensive effect both at 20 and 40 mg as compared to placebo. There was no significant change in heart rate with 20 mg of nitrendipine, whereas there was a significant increase in heart rate after the first dose of 40 mg. Nitrendipine in dosages of 20 mg and 40 mg daily was equally effective as regards the antihypertensive effect. This applies both to the early response (first day) and the late response (3 weeks). Side effects were common in the 40-mg treatment group, but rare in the 20-mg group; only one patient had headache (three times) during the 3-week treatment period. It can be concluded that in mild hypertension, treatment with 20 mg of nitrendipine once daily constitutes an effective antihypertensive therapy. In more severe cases of hypertension, however, combination treatment with beta-blockers, for example, and/or repeated daily administration of nitrendipine ought to be investigated.