His bundle electrograms were recorded by a signal averaging technique using the Takayasu vectorial lead system, and effects of ouabain on the atrioventricular conduction were investigated using Wistar rats. Ouabain was given intraperitoneally at 30 min intervals in doses of 1-40 mg/kg. When ouabain dosage was increased to 20 mg/kg, the prolongation of PQ interval became prominent and P waves were hidden on the preceding T waves. Dose-related prolongation of PQ and AH intervals was observed in the 1-15 mg/kg dosages. Ouabain-induced ventricular arrhythmias occurred in the 30 mg/kg dosage. Dose-related prolongation of QRS duration and HV interval was observed in the 1-25 mg/kg dosages. These prolongations were significant by the analysis of variance (p less than 0.01). Prolongation of PQ and AH intervals is probably due to a digitalis-induced increase in parasympathetic nerve activity. However, the reason for the prolongation of HV interval and QRS duration was uncertain. Direct effects of ouabain on the conduction system, or central nervous system effects of ouabain on cardiovascular function were speculated. These results suggest the applicability of the noninvasive record of His bundle electrogram to assessing the digitalis effect even on the human atrioventricular conduction.