Electrophysiological properties were studied in a heterotopic heart transplant model developed in 44 dogs (Group I). Cycle length was 359.5 +/- 55.2 ms in the recipient heart and 500.9 +/- 77.9 ms in the donor heart (P less than 0.001). Sinoatrial conduction time was 38.6 +/- 13.6 ms in the recipient heart and 38.6 +/- 10.4 ms in that of the donor (not significant). The Wenckebach point was 175.4 +/- 31.1 ms in the recipient organ and 214.3 +/- 42.6 ms in the donor heart (P less than 0.001). The retrograde block point was 271.6 +/- 48.0 ms in the recipient heart and 353.6 +/- 47.3 ms in that of the donor (P less than 0.01). The effective antegrade refractory period was 133.4 +/- 28.7 ms in the recipient heart and 167.3 +/- 46.3 ms in the donor organ (P less than 0.001). An epicardial atrial cartography was performed at 44 preset points in both hearts, revealing a normal activation sequence and delays of 36-98 ms (mean 58 ms) in the recipient heart and from 39 to 59 ms (mean 50 ms) in the donor heart (not significant). In another 11 dogs (Group II), the same electrophysiological study was carried out under basal conditions and after pharmacological blockade of the autonomic nervous system with atropine (0.04 mg (kg body weight)-1) and propranolol (0.2 mg (kg body weight)-1). No significant differences were found in any of the parameters studied upon comparison of Group II animals in basal conditions with Group I recipients, and Group II dogs after blockade with Group I donors. These results demonstrate that the differences in automatism, conduction and refractoriness between the donor and recipient hearts are not attributable to differences in the haemodynamic situation or in the anaesthetic technique, but to denervation.