Calcium antagonists are known to have an inhibitory effect on the contractile status of the smooth muscle of the arterioles, the muscle fibers of the heart, and the cardiac conduction system, especially the atrioventricular node. The vasodilatation that is produced may be observed in the coronary arteries but predominates in the peripheral resistance vessels. Nifedipine and its 1,4-dihydropyridine derivative, nitrendipine, are not only useful in coronary artery disease but also have potential as antihypertensive agents. The acute hemodynamic effects of both nifedipine and nitrendipine, administered in a 1.0 mg intravenous dose, were studied. The compounds were randomly allocated to six patients, where each served as his own control. In addition, the effect of 1 mg of nitrendipine intravenously on the electrophysiological impulse formation and conduction was investigated in a second group of six patients. There were substantial differences in the intensity and duration of the hemodynamic effects of both drugs. The total peripheral resistance (TPR) fell after nifedipine from 1,730 +/- 240 to 1,280 +/- 120 and after nitrendipine from 1,960 +/- 420 to 1,200 +/- 150 dynes-s-cm-5. There was a concurrent significant reduction in the systolic aortic and left ventricular pressure after nifedipine and nitrendipine from 152 +/- 8 to 128 +/- 13 and 161 +/- 19 to 122 +/- 18 mm Hg, respectively. Heart rate increased due to baroreceptor stimulation by 21% with nifedipine and by 32% with nitrendipine. From the results of the electrophysiological study, nitrendipine did not have any significant effect on the sinus or atrioventricular node function. The substance may be safely used in patients with conduction disturbances.