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 (ADPbetaS), unaffected by desensitisation with P2X receptor agonist, alpha,beta-methylene ATP (alpha,beta-meATP), and reduced by PACAP-(6-38), a PACAP receptor antagonist. ATP induced membrane hyperpolarization resistant to tetrodotoxin, N(omega)-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.