Neuropeptide Y (NPY) attenuates vagal effects on cardiac cycle length, presumably by inhibiting the release of acetylcholine from vagal nerve endings. We sought to determine if NPY inhibited the vagal effects on atrioventricular (AV) interval and atrial contraction in a manner similar to its inhibition of the vagal effects on cycle length. In 19 anesthetized dogs we measured the vagal effects on cycle length, AV interval, and atrial contraction before and after 3-minute trains of sympathetic stimulation or before and after exogenous NPY (20 micrograms/kg i.v.). Three minutes after 10-Hz sympathetic stimulation, the vagal effects on cycle length and AV interval were attenuated by 52 +/- 9% and 63 +/- 8%, respectively. Phentolamine significantly augmented this attenuation, but propranolol had no appreciable effect. In the control group of animals or in the group that received phentolamine, the vagal effects on atrial contraction measured before and after sympathetic stimulation were not significantly different. In these two groups, however, the basal atrial contraction was reduced substantially after the cessation of sympathetic stimulation. Propranolol prevented this reduction in atrial contraction. After propranolol, the vagal effects on atrial contraction 3 minutes after sympathetic stimulation were attenuated by 31 +/- 6%. Exogenous NPY had no direct effect on cycle length, AV interval, or atrial contraction, but exogenous NPY did persistently inhibit the vagal effects on each of these cardiac processes. Three minutes after NPY was given, the vagal effects on cycle length, AV interval, and atrial contraction were inhibited by 62 +/- 7%, 69 +/- 5%, and 68 +/- 5%, respectively. We conclude that NPY attenuates the vagal effects on the atrial myocardium and on the sinus and AV nodes. In the absence of beta-blockade, the inhibitory effect of neurally released NPY on the vagally induced decreases in atrial contraction may be masked by the reduction in the atrial contraction that occurs after sympathetic stimulation.