This study examined the effects and interactions evoked by various combinations of parasympathetic (P) and sympathetic (S) stimulations on sinus node automaticity and atrioventricular (AV) conduction (both anterograde and retrograde) in 11 2-mo-old anesthetized puppies. The respective effects of these autonomic maneuvers were assessed by covariant analysis using a linear regression model of the form Y = α0 + α1 x base line + α2 x sequence + α3 x S + α4 x S2 + α5 x P + α6 x P2 + α7 x (P x S).... The effects of parasympathetic and sympathetic stimulation, of the quadratic term S2, and of the interaction term (P x S) were highly significant (P < 0.0001) on both sinus node automaticity and AV conduction (anterograde and retrograde). In contrast, the effect of the quadratic term P2 was significant on sinus node automaticity only. Sequence of stimulation whether (SP) or (PS) had no significant effect on either chronotropic or dromotropic properties. Furthermore, whereas augmented antagonism with parasympathetic predominance over sympathetic activity was readily demonstrable in the control of sinus rate, the reverse, i.e., augmented antagonism with sympathetic preponderance over parasympathetic activity, was found to apply to the control of AV conduction (both anterograde and retrograde). Hence, the nature and extent of parasympathetic-sympathetic interactions vary considerably, depending on which cardiac structure and/or function is under scrutiny. It appears that AV conduction is especially sensitive to modulation of sympathoadrenergic activity, whereas sinus node automaticity is particularly responsive to cholinomimetic influences.