Vagal effects on atrioventricular (AV) nodal conduction are accentuated by increases in heart rate. To establish the mechanism of these rate-dependent negative dromotropic actions, we studied the properties governing AV nodal adaptation to changes in heart rate in chloralose-anesthetized dogs in the absence and presence of bilateral cervical vagal nerve stimulation (20 Hz, 0.2 msec). Stimulation protocols were applied to evaluate the contributions of changes in AV nodal recovery, facilitation, and fatigue independently of each other. Vagal stimulation slowed AV nodal recovery in a voltage-dependent way, increasing the time constant of recovery (tau r) from 80 +/- 7 to 194 +/- 16 msec (mean +/- SEM, p less than 0.01) at the highest voltage studied. The facilitating effect of a premature (A2) beat was manifested by a leftward shift of the recovery curve (A3H3 versus H2A3) of a subsequent A3 beat. The magnitude of shift depended on the A1A2 coupling interval and was reduced by vagal stimulation at all A1A2 intervals (maximum shift: control, 63 +/- 12 msec; vagus, 24 +/- 11 msec; p less than 0.01). When recovery and facilitation were kept constant, abrupt increases in AV nodal activation rate caused a slow (tau = 75 beats) increase in AH interval (fatigue). Vagal stimulation increased the magnitude of this process (maximum: control, 11 +/- 2 msec; vagus, 27 +/- 3 msec; p less than 0.001), without altering its time course. At activation rates comparable to sinus rhythm in humans, vagal stimulation at an intermediate voltage increased the AH interval by 25 msec. As heart rate increased, vagally induced changes in dynamic processes amplified AH prolongation up to fivefold at maximum rate. The role of vagal changes in individual functional properties depended on heart rate, but slowing of recovery was the single most important factor, constituting over 50% of overall vagal action at rapid rates. We conclude that vagal stimulation alters the ways in which the AV node responds to changes in activation rate and that at rapid rates most of the negative dromotropic action of the vagus is due to changes in the AV nodal response to tachycardia. Alterations in rate-dependent AV nodal properties are a novel and potentially important mechanism through which interventions may affect AV nodal conduction.