The effects of postganglionic vagal stimulation on atrioventricular nodal conduction were studied in 12 rabbit atrial-atrioventricular nodal preparations. Vagal stimulation was introduced in the sinus and atrioventricular nodes, separately or in combination, using single bursts of subthreshold stimuli. The sinus cycle length was scanned to identify the phasic effect of vagal stimulation. Action potentials from cells in the AN, N, and NH regions of the atrioventricular node were recorded by microelectrode techniques. Vagally induced hyperpolarization of cells in the atrioventricular node resulted in a phase-dependent prolongation of conduction time and reflected the level of residual hyperpolarization at the moment of arrival of the next atrial beat at the atrioventricular nodal input region. Vagally induced hyperpolarization was membrane potential dependent, although its overall time course was similar at different phases. Increased diastolic depolarization followed the maximal hyperpolarization. This "rebound" observed at certain phases was responsible for paradoxical shortening of the conduction time after vagal stimulation. The predominant effects of local vagal stimulation in the atrioventricular node were observed in cells in or near the N region. Slower rate of rise, shorter amplitude and duration, as well as step formations were among the changes in action potentials recorded from these cells. The effects of vagal stimulation were inhomogeneous between different regions of the atrioventricular node as well as within the N region, producing alternative pathways of conduction and the potential for reentry. The concomitant changes in sinus cycle length resulting from vagal stimulation in the sinus node region altered the phasic effects of vagal stimulation introduced in the atrioventricular node. This was related to a direct influence of the prolonged sinus cycle length on atrioventricular nodal refractoriness as well as an indirect effect on the degree of residual vagally induced hyperpolarization at the moment of arrival of the delayed atrial beat. These findings provide mechanistic explanations for the complex effects of vagal stimulation on atrioventricular nodal conduction.