To investigate the potential gradient threshold in different directions to excite cardiac tissue, perfused rabbit papillary muscle was used and transmembrane potential was recorded when rectangular pulses were applied to the tissue. Electrical pulses were produced through large stimulating electrodes to create an even potential gradient field, the direction of which was parallel or perpendicular to the long axis of the papillary muscle. The thresholds for excitation were found to be 0.60±0.31 V/cm and 1.02± 0.67 of the potential gradient along and across the fiber orientation respectely, with a ratio of 1:1.73 (P<0.01, n=6). Thus, the dependence of cardiac tissue excitation threshold on the direction of the potential gradient has been clearly demonstrated, the relationship of which to the electrical ventricular defibrillation and the occurrence of anisotropic reentry have both been discussed.