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The guinea-pig ileum longitudinal muscle-myenteric plexus preparation, pre-incubated with [3H]choline, was mounted in an organ bath and superfused with Tyrode's solution. [3H]Acetylcholine secretion was evoked by 150 electrical shocks at 0.5 Hz. >N6,2′-0-Dibutyryladenosine 3′,5′-cyclic monophosphate (dibutyryl cyclic AMP) enhanced the [3H]acetylcholine secretion in the presence of eserine and the adenosine receptor antagonist 8-phenyltheophylline (10 μmol 1-1). Conversely, in the absence of 8-phenyltheophylline the [3H]acetylcholine secretion was reduced by dibutyryl cyclic AMP. In the absence and presence of 8-phenyltheophylline (apparent KD = 12 μ.mol 1-1), adenosine reduced the [3H]acetylcholine secretion to 33% of control (IC50 = 8, μmol l-1) and to 48% of control (IC50 = 14 μmol l-1) respectively. Neither butyrate, dibutyryl cyclic GMP nor guanosine altered the [3H]acetylcholine secretion. Interaction experiments with 3-isobutyl-1-methylxanthine and oxotremorine were done in the absence of eserine, i.e. when oxotremorine is effective. Oxotremorine depressed the fractional secretion of [3H]acetylcholine with a ‘maximal inhibition’ of 13% of control (IC50 = 10 nmol l-1). In the presence of 3-isobutyl-1-methylxanthine (5 mmol l-1) oxotremorine depressed the secretion to 2% of control with an apparent IC50) value of 0.9 μmol l-1 3-Isobutyl-I-methylxanthine (0.01–4 mmol l-1) enhanced the fractional secretion of [3H]acetylcholine with a ‘maximal enhancement’ value of 232% of control (EC50 = 0.19 mmol l-1). The presence of oxotremorine (30 nmol l-1) counteracted, and higher concentrations reversed, the enhancement caused by 3–isobutyl-I-methylxanthine. The results are consistent with the suggestion that the presynaptic muscarinic acetylcholine receptor-mediated control of the secretion of [3H]acetylcholine in the guinea-pig ileum myenteric plexus may be mediated both by cyclic AMP and calcium ions.