We investigated, in murine colon circular muscle, the role of adenosine 5′-triphosphate (ATP) and pituitary adenylate cyclase activating peptide (PACAP) as inhibitory neurotransmitters of the fast component of nerve-evoked inhibitory junction potential (fast IJP). Fast IJP was antagonised by apamin or suramin, abolished by desensitisation with the P2Y receptor agonist, adenosine 5′- O-2-thiodiphosphate (ADPβS), unaffected by desensitisation with P2X receptor agonist, α,β-methylene ATP (α,β-meATP), and reduced by PACAP-(6–38), a PACAP receptor antagonist. ATP induced membrane hyperpolarization resistant to tetrodotoxin, N ω-nitro- l-arginine methyl ester ( l-NAME) or PACAP-(6–38), but antagonised by apamin, suramin, P2X and P2Y receptor desensitisation. PACAP-(1–27) caused membrane hyperpolarization antagonised by PACAP-(6–38), apamin and P2Y receptor desensitisation, reduced by tetrodotoxin, but not affected by l-NAME and by P2X receptor desensitisation. Therefore, in murine colon circular muscle, an ATP-like endogenous P2Y purinoceptor ligand is the final nonadrenergic, noncholinergic (NANC) inhibitory mediator involved in the generation of fast IJP. A neuromodulator role of PACAP in the inhibitory pathway is supposed.