In gastrointestinal tissues such as rat stomach, exogenous 5-hydroxytryptamine (5HT) has little or no ability to affect nerve activity. However, endogenous 5HT might act differently, and this was investigated by stimulating 5HT synthesis using 5-hydroxy-L-tryptophan (5HTP). In longitudinal strips of rat forestomach, 5HTP (50 and 500 microM) increased cholinergically mediated contractions evoked by electrical field stimulation, probably by facilitating acetylcholine release; contractions evoked by exogenous acetylcholine were unaffected by 5HTP. The ability of 5HTP to increase electrically evoked contractions was long-lasting, required the presence of pyridoxal (a monoamine decarboxylase cofactor) and was reduced by the decarboxylase inhibitor carbidopa, but not by 6-hydroxydopamine. In the presence of paroxetine and nialamide, the 5HTP-induced increase in cholinergically mediated contractions was short-lasting. In anaesthetised rats 5HTP caused stimulation of gastric motility, which was blocked or reduced by atropine. These findings suggest that in the rat 5HTP stimulates gastric cholinergic activity, by increasing the concentration of 5HT at sites which normally synthesise 5HT.