The experiments were performed to study the effects of Flavoxate hydrochloride on the gastrointestinal motility and its underlying mechanism. Gastrointestinal motility was inhibited at first and then accelerated markedly after the intravenous injection of Flavoxate hydrochloride (10 mg/kg) in the experiments of anesthetized dogs. For analyzing the underlying mechanism of this response the following experiments were executed. 1) The pendular movements of isolated ileum of rabbits were accelerated with administration of low concentration of Flavoxate hydrochloride less than 10(5) g/ml, while these were reduced with administration of high concentration more than 10(4) g/ml. Because the tetrodotoxin (2.5 times 10(7) g/ml) has no essential effect on these responses, it could be postulated that Flavoxate hydrochloride has direct effect on intestinal smooth muscle itself. 2) Flavoxate hydrochloride (10 mg/kg) induced the differentiated regional response of the sympathetic outflow, that is, the parallel decrease of cutaneous and cardiac sympathetic activities, and the converse increase of splanchnic activity. Therefore, Flavoxate hydrochloride is thought to have an effect on the integrating mechanism of the sympathetic nervous system. This view was also supported by the experiments with neuromuscular preparation of lobster (Panulirus japonicus). It is concluded from these results, that the change of gastrointestinal motility induced by intravenous injection of Flavoxate hydrochloride (10 mg/kg) is influenced both by its effect on the gastrointestinal smooth muscle itself and by its effect via the autonomic nervous system.