In a randomized parallel-group placebo-controlled study, we compared the short-term hypotensive efficacy and the safety of a single administration of nifedipine-retard (20-mg tablets) with that of two administrations 6 h apart of nifedipine capsules (10 mg) in 10 and 11 black patients, respectively, with acute severe hypertension. Both groups had similar pretreatment blood-pressure (BP) values. Blood pressure was recorded at 10-min intervals for 12 h by using an automated device. In the first 3 h of treatment, nifedipine capsules induced a faster and greater hypotensive effect than nifedipine retard, which was associated with an increase in heart rate. At 2 h after treatment, nifedipine capsules decreased BP to levels (159 +/- 5/105 +/- 3 mm Hg) that were significantly lower than those reached by nifedipine-retard (175 +/- 4/118 +/- 4 mm Hg; p < 0.05). Both preparations induced a similar maximal BP decrease of approximately 30% of the placebo values, but the peak decrease of BP occurred significantly later with nifedipine-retard (283 +/- 31 min after administration) than with nifedipine capsules (100 +/- 14 min; p < 0.01). Four hours after administration, the hypotensive effect of nifedipine capsules was blunted, and a second administration was necessary, whereas nifedipine-retard reduced BP slowly and continuously for < or =12 h and more smoothly. Flush and headache were more frequently found with nifedipine capsules. We conclude that in black patients with hypertensive crisis, nifedipine capsules produce an abrupt decrease in BP that may be potentially harmful. Thus for patients suitable for treatment with nifedipine, nifedipine-retard is preferable because it effectively reduces BP for > or =12 h while achieving a rapid enough effect without critical short-term decreases in BP.