ABSTRACTConcurrent stimulation of the parasympathetic and sympathetic branches of the autonomic nervous system causes a diminished sympathetic response at high levels of vagal activity. This "accentuated antagonism" has been demonstrated for cardiac chronotropic, inotropic, and dromotropic responses. The effect on conduction was demonstrated with tonic stimulation of the vagus nerve. However, normally the vagus nerve fires periodically at certain times in the cardiac cycle. Thus, we have studied whether a similar interaction exists in the modulation of atrioventricular condition when short bursts of vagal stimulation were placed in various portions of the cardiac cycle. Anesthetized open-chest mongrel dogs were instrumented for stimulation of the cervical vagi and stellate ganglia when the heart was paced. We determined the relationship between cardiac cycle length, direction of action potential propagation, and levels of sympathetic and vagal activation and their effects on atrioventricular conduction times. All of the factors investigated, namely levels of vagal and stellate stimulation, pacing intervals, and direction of propagation of action potentials, affected atrioventricular conduction times. Furthermore, the vagal effect was greater at short cardiac cycle lengths. When bursts of vagal stimulation were timed to result in maximal or minimal prolongation of atrioventricular conduction, no significant effects of sympathetic-parasympathetic interaction on atrioventricular conduction times were apparent. However, an analysis of the differences in prolongation of atrioventricular conduction with periodic vagal stimulation revealed that a significant sympathetic-vagal interaction existed for these differences. Thus, autonomic neurotransmitters differentially affect cardiac conduction times depending on time of application of the stimulus.