The cellular electrophysiological effects of 1 microM tedisamil (KC 8857) were studied in human atrial and ventricular fibers. Conventional microelectrode technique was applied to record the transmembrane action potentials at stimulation frequency of 100 per min and 37 degrees C. Tedisamil lengthened action potential duration (APD) more in atrial than in ventricular muscle fibers; prolongation of APD90 was 28.9 +/- 3.3% (n = 6; p < 0.05) for atrial and 13.3 +/- 5.2% (n = 6; P < 0.05) for ventricular tissue. The maximal rate of depolarization was depressed slightly, but significantly by 1 microM tedisamil only in ventricular fibers (12.9 +/- 6.5%, n = 6, P < 0.05). From these cellular electrophysiological data it is concluded that the bradycardic/antiischemic agent tedisamil possesses marked Class III properties not only in cardiac tissues of experimental animals but also those of man.