To evaluate the chronobiologic pattern of the hypotensive effect of nitrendipine, 10 patients with mild-to-moderate arterial hypertension were studied. They received a randomized single dose (20 mg) of nitrendipine and placebo, and 20 mg of nitrendipine daily for 2 months. Systolic and diastolic blood pressure (SBP and DBP, respectively) and heart rate (HR) were measured for 24 h using an automatic noninvasive device. The data were analyzed by single and mean cosinor methods and by ANOVA and Student's paired t test. Chronic administration of nitrendipine resulted in a more effective lowering of the SBP and DBP mesor, compared with placebo and acute administration, preserving the circadian rhythms. The preservation of the HR circadian rhythm agrees with the lack of interference of the drug with neurohormonal mechanisms.