Effects of growth hormone-releasing hormone (GRH) and corticotropin-releasing hormone (CRH) on the release of immunoreactive thyrotropin-releasing hormone (ir-TRH) from the rat hypothalamus in vitro were studied. The rat hypothalamus was incubated in medium 199 with 1.0 mg/ml of bacitracin (pH 7.4) for 20 min. The amount of ir-TRH release into the medium was measured by radioimmunoassay. The ir-TRH release from the rat hypothalamus was inhibited significantly in a dose-related manner with the addition of GRH or CRH. These findings suggest that GRH and CRH inhibit ir-TRH release from the rat hypothalamus in vitro.