The effect of neuropeptide Y on the motor activity of the circular and longitudinal muscle of the guinea pig small intestine was investigated. Neuropeptide Y (0.3-30 nM) inhibited, in a concentration-dependent manner, the ascending enteric reflex contraction of the circular muscle; both the cholinergic and noncholinergic component of the ascending enteric reflex were suppressed. Neuropeptide Y also blocked nerve-mediated phasic contractions of the circular muscle, induced by dimethylphenylpiperazinium or ceruletide, and abolished the peristaltic movements of the small intestine. Nerve-mediated cholinergic and noncholinergic contractions of the longitudinal muscle, induced by electrical field stimulation, were only slightly depressed by neuropeptide Y. Further analysis showed that neuropeptide Y did not act directly on intestinal muscle but interrupted excitatory pathways of the enteric nervous system. The effect of neuropeptide Y did not involve activation of alpha-adrenoceptors, opioid receptors, or P1 purinoceptors or the release of endogenous factors stimulating these receptors. These findings suggest a possible physiologic role of neuropeptide Y as a nonnoradrenergic inhibitory transmitter involved in the enteric nervous control of intestinal circular muscle activity.