The vagal nerves modulate adrenergic effects on sinus cycle length, atrioventricular (AV) nodal conduction, and refractoriness of atria and ventricles. We tested whether varying levels of vagalsympathetic input could yield the same spontaneous sinus cycle length but also alter effective refractory periods of the right atrium, right ventricle, and left ventricle and AV nodal conduction times. Dogs anesthetized by α-chloralose were studied in the open-chest, neurally decentralized state. In 10 dogs, sinus cycle length was maintained constant during 3 levels of bilateral ansae subclaviae stimulation (4 msec, 3 mA at 1, 2, and 4 Hz) by titrating simultaneous bilateral vagal stimulation (varying pulse width and frequency). Each combination of ansae subclaviae-vagal stimulation yielded the same sinus cycle length as the control value, but refractory periods of right atrium and right and left ventricles shortened progressively as the frequency of ansae subclaviae stimulation increased. Atrioventricular nodal conduction time (AH interval) shortened in 2 dogs and lengthened in 3 dogs. His-Purkinje conduction time (HV interval) was unchanged. In 9 dogs, the effects of simultaneous unilateral ansae subclaviae stimulation at 2 Hz and ipsilateral vagal stimulation that yielded the same sinus cycle length were determined. Right-sided ansae subclaviae–vagal stimulation shortened refractoriness of right atrium and anterior left ventricle significantly. The AH interval lengthened in 1 dog. Left ansae subclaviae–vagal stimulation shortened the refractory periods of anterior and posterior left ventricle significantly and reduced the AH interval in 3 dogs. In 8 dogs, the effects of bilateral ansae subclaviae stimulation alone at 2 Hz, vagal stimulation alone at an intensity required to keep the sinus cycle length constant during ansae subclaviae stimulation, and simultaneous bilateral ansae subclaviae and vagal stimulation were tested. The right atrial refractory period was shortened significantly by ansae subclaviae stimulation alone and by vagal stimulation alone and was shortened further by simultaneous stimulation of both autonomic limbs. The right and left ventricular refractory periods were shortened by ansae subclaviae stimulation alone and by simultaneous stimulation of both limbs but tended to be prolonged by vagal stimulation alone and when added to ansae subclaviae stimulation. In 7 dogs, the effects of simultaneous stimulation of bilateral ansae subclaviae at 2 Hz and vagi at intensities that maintained the AH interval constant at an atrial pacing cycle length of 300 msec were determined. During these stimulation parameters, the sinus cycle length was prolonged by 90 msec or more in 3 dogs and varied within 15 msec in the remaining 4 dogs, while refractoriness of atrium and ventricle shortened significantly. We conclude that multiple combinations of ansae subclaviae–vagal stimulation that do not alter the spontaneous sinus cycle length can lengthen or shorten AV nodal conduction time and can shorten atrial and ventricular refractory periods. Under these conditions, sinus cycle length is not an indicator of autonomic input to the AV node, atrium, and ventricle of the canine heart. Similarly, under autonomic conditions that yielded a constant AH interval, atrial and ventricular refractory periods shortened and spontaneous sinus cycle length lengthened or shortened.