Segments from various locations of the small and large intestine of the rat were removed, bathed in Tyrode's solution and attached to a force displacement transducer. Melatonin, while not influencing the frequency of contraction, did reduce the force of spontaneous contractions of duodenal and colon segments of rat intestine by 92 and 52%, respectively compared to only 25 and 22% for the ileum and jejunum, respectively. Areas with greatest responsiveness to melatonin were those that previous studies have shown to contain the largest concentrations of endogenous melatonin. Cyclic guanosine monophosphate, when tested in similar preparations, did not produce an inhibitory response characteristic of melatonin. It is hypothesized, therefore, that this hormone has physiological action within the gut, including motility; however, its action may not be directly on smooth muscle contraction but may be through an indirect action inhibiting the contractile response of serotonin, as suggested by other investigators.