Actions of adenosine 5'-monophosphate (AMP) on electrical and synaptic behavior of submucosal neurons in guinea-pig small intestine were studied with "sharp" intracellular microelectrodes. Application of AMP (0.3-100 µM) evoked slowly-activating depolarizing responses associated with increased excitability in 80.5% of the neurons. The responses were concentration-dependent with an EC₅₀ of 3.5 ± 0.5 µM. They were abolished by the adenosine A_2A receptor antagonist, ZM-241385, but not by PPADS, TNP-ATP, 8-CPT, suramin or MRS-12201220. The AMP-evoked responses were insensitive to AACOCF3 or ryanodine. They were reduced significantly by: 1) U-73122, which is a phospholipase C inhibitor; 2) cyclopiazonic acid, which blocks the Ca²⁺ pump in intraneuronal membranes; 3) 2-Aminoethoxy-diphenylborane, which is an IP3 receptor antagonist. Inhibitors of protein kinase C or calmodulin-dependent protein kinase also suppressed the AMP-evoked excitatory responses. Exposure to AMP suppressed fast nicotinic ionotropic postsynaptic potentials, slow metabotropic excitatory postsynaptic potentials and slow noradrenergic inhibitory postsynaptic potentials in the submucosal plexus. Inhibition of each form of synaptic transmission reflected action at presynaptic inhibitory adenosine A₁ receptors. Slow excitatory postsynaptic potentials, which were mediated by the release of ATP and stimulation of P2Y₁ purinergic receptors in the submucosal plexus, were not suppressed by AMP. The results suggest an excitatory action of AMP at adenosine A_2A receptors on neuronal cell bodies and presynaptic inhibitory actions mediated by adenosine A₁ receptors for most forms of neurotransmission in the submucosal plexus, with the exception of slow excitatory purinergic transmission mediated by the P2Y₁ receptor subtype.