Effects of 8 synthetic tripeptides--structural analogues of thyroliberin (TRH)--in basal and TRH-induced secretion of prolactin (PRL) and thyrotropin (TSH) were studied in primary cultures of rat pituitary cells. 7 tripeptides had chemical structure Pyr-Ser-X-NH2, where X was Leu, Val, Asn, Ala, Ile, Met, Phe. One tripeptide had the structure Pyr-Ser-Gly-OCH3. Some of the tripeptides studied exhibited weak TRH-like effects on PRL and TSH release and other substances were able to inhibit partially the TRH-induced PRL secretion. However, none of the TRH analogues studied changed secretory activity of thyrotropic and lactotropic cells in culture in unidirectional manner. The data obtained suggest heterogeneity of the TRH receptors in lactotrophs and thyrotrophs of rat pituitary.