In the present study, the effect of TRH on amylase secretion was determined both in vivo, by cannulating the pancreatic duct of rats, as well as in vitro, by using isolated lobules and dissociated acini. The results show that TRH inhibited both basal and stimulated in vivo amylase secretion. Nevertheless, the in vitro experiments failed to show a TRH-related inhibitory effect when TRH was used alone, although the hormone did blunt the secretion elicited by CCK8 and bethanechol from isolated lobules and dissociated acini. Results suggest that TRH can inhibit stimulated amylase secretion in rats through a direct effect on acinar cells.