A tablet form of nifedipine was given to eight hypertensive hospitalized men (Stage I or II WHO, 45 +/- 10 years old). After an initial placebo test, 20, 40, and 60 mg of nifedipine were given at 8.00 a.m. in random order at 72-hour intervals in a single administration double-blind crossover study. Blood pressure and heart rate were measured twice by the same observer every 20 minutes from 7.00 a.m. to 8.00 a.m. and then hourly until 8.00 p.m., first with the patients recumbent and again after 1 minute of standing. Plasma nifedipine levels were assayed in samples drawn hourly from 8.00 a.m. to noon, every 2 hours from noon to 8.00 p.m., and at 24 and 48 hours after drug ingestion. The three doses all lowered blood pressure significantly. The reduction during recumbency was significantly larger (-18%) and lasted longer (12 hours) after 60 mg than after 20 mg (-11% at 7 hours). The three doses caused similar increases in heart rate (+29% to +38%), the maximum occurring at the second hour and lasting for 5 hours. The peak plasma concentrations and areas under the plasma concentration time curve were dose-dependent; kinetics were linear between 20 and 60 mg, and the half-life of nifedipine tablets was close to 10 hours. The decrease in mean arterial blood pressure correlated strongly with plasma nifedipine levels (r = 0.61; n = 190; p less than 0.001). Four patients experienced mild side effects (headaches, flushes, drowsiness, or weakness). The tablet form of nifedipine had a potent antihypertensive action that lasted longer than that of the capsule formulation.