1. The actions of adenosine on electrical behaviour of myenteric neurones were investigated with intracellular recording methods in guinea-pig small intestine. 2. The actions of adenosine were: membrane hyperpolarization, decreased input resistance, enhancement of post-spike hyperpolarizing potentials and suppression of excitability. These effects were observed exclusively in AH/type 2 myenteric neurones. 3. The presence of adenosine in the micromolar range of concentrations prevented or suppressed excitatory responses to forskolin. It also aborted the effects of forskolin when added in combination with this activator of adenylate cyclase. 4. Adenosine (100 microM) did not affect the excitatory actions of intracellularly injected cyclic AMP, membrane-permeant analogues of cyclic AMP, inhibitors of cyclic nucleotide phosphodiesterase or elevation of Mg2+ and reduction of Ca2+ in the bathing medium. 5. The results suggest that the mechanism of the inhibitory action of adenosine is suppression of the catalytic activity of adenylate cyclase and consequent reduction of intraneuronal levels of cyclic AMP.