The effect of endogenous opioid peptides (beta-endorphin, leucine-enkephalin, dynorphin 1-13 on the release of thyrotropin releasing hormone (TRH) from the rat stomach in vitro was studied. The rat stomach was incubated in the medium 199 with 1.0 mg/ml of bacitracin (pH 7.4) (medium) and the amount of TRH released into the medium was measured by radioimmunoassay. The release of TRH from the rat stomach was inhibited significantly in a dose-related manner with the addition of endogenous opioid peptides, but not affected with naloxone. However, the inhibitory effect of endogenous opioid peptides on TRH release from the rat stomach was prevented by the addition of naloxone. These findings suggest that endogenous opioid peptides inhibit TRH release from the rat stomach in vitro.