Correlation between the effective refractory period of the conduction system and the histopathologic grade of the ventricle was examined in the rat cardiac allograft during acute rejection. Lewis rats were recipients of intraabdominal heart grafts from brown Norway rats (allogeneic group, n = 42) or Lewis rats (syngeneic group, n = 15). No immunosuppressant was given. The effective refractory period of the conduction system was measured by the programmed atrial extrastimulus method (basic cycle length, 150 msec) just before the time of death. Specimens of the transplanted hearts were examined histopathologically and the histopathologic grade of rejection was scored according to the standardized grading system of the International Society for Heart and Lung Transplantation. The effective refractory period of the allogeneic heart was significantly longer starting on the third day after transplantation (p less than 0.01). The effective refractory period of the allogeneic heart was more prolonged as the severity of rejection increased. The correlation between the effective refractory period (Y) and the histopathologic grade (X) of the allogeneic group was statistically significant (r = 0.955; Y = 10.3X + 81.0; p less than 0.01). The effective refractory period of all syngeneic hearts and allogeneic hearts of postoperative day 1 and 2 were statistically equivalent to the period of native rat hearts (81.0 +/- 2.0 msec; n = 6). The histopathologic grade of the conduction system was the same as that of the ventricle. We conclude that the effective refractory period of the conduction system could be a useful measure to predict the histopathologic grade of cardiac allograft rejection.