Intracellular recordings were made from myenteric neurons of the guinea-pig ileum in vitro; they were classified into S and AH neurons according to electrophysiological criteria. ATP (10 nM-100 microM) inhibited excitatory synaptic potentials in the myenteric plexus; fast excitatory postsynaptic potentials and slow excitatory postsynaptic potentials of S neurons and slow excitatory postsynaptic potentials in AH neurons. This inhibitory action was reversible and dose-dependent, and was usually followed by a transient augmentation of the synaptic potentials after washing of ATP. The actions of ATP on the synaptic potentials were prevented by pretreatment with theophylline, caffeine, quinidine and 8-phenyl theophylline. The ATP analogues, ATP-gamma-s (100 nM-100 microM) and alpha-beta-methylene ATP (100 nM-100 microM) also depressed the synaptic potentials recorded from both types of neurons. The inhibitory effect of adenosine on the synaptic potentials was 10 times weaker than that of ATP. Thus, it seems clear that the presynaptic inhibition is not occurring through adenosine A1 or A2 receptors. Furthermore, ATP at high concentrations ( > or = 1 microM) augmented nicotinic fast depolarizations of S neurons produced by extracellular acetylcholine. However, ATP at the same concentrations inhibited the slow depolarizations of S and AH neurons caused by exogenous acetylcholine (muscarinic) and substance P. It is concluded that ATP regulates synaptic transmission in the myenteric plexus of the guinea-pig ileum and the sites of ATP actions are pre- and postsynaptic.