Intracellular recordings were made from the circular smooth muscle cells of the canine jejunum to study the effect of exogenous ATP and to compare the ATP response to the nonadrenergic, noncholinergic (NANC) inhibitory junction potential (IJP) evoked by electrical field stimulation (EFS). Under NANC conditions, exogenous ATP evoked a transient hyperpolarization (6.5 ± 0.6 mV) and EFS evoked a NANC IJP (17 ± 0.4 mV). -Conotoxin GVIA (100 nM) and a low-Ca²⁺, high-Mg²⁺ solution abolished the NANC IJP but had no effect on the ATP-evoked hyperpolarization. The ATP-evoked hyperpolarization and the NANC IJP were abolished by apamin (1 µM) and N^G-nitro-L-arginine (100 µM). Oxyhemoglobin (5 µM) partially (38.8 ± 5.5%) reduced the amplitude of the NANC IJP but had no effect on the ATP-evoked hyperpolarization. Neither the NANC IJP nor the ATP-evoked hyperpolarization was affected by P2 receptor antagonists or agonists, including suramin, reactive blue 2, 1-(N,O-bis-[5-isoquinolinesulfonyl]-N-methyl-L-tyrosyl)-4-phenylpiperazine, pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid, ,-methylene ATP, 2-methylthioadenosine 5'-triphosphate tetrasodium salt, and adenosine 5'-O-2-thiodiphosphate. The data suggest that ATP evoked an apamin-sensitive hyperpolarization in circular smooth muscle cells of the canine jejunum via local production of NO in a postsynaptic target cell.