The role of endogenous luteinizing hormone-releasing hormone (LHRH) in the development of concanavalin A (ConA)-induced proliferative responses was studied in rat fetuses. Preliminary treatment of fetuses in utero with either the LHRH receptor antagonist or anti-LHRH antibodies resulted in the suppression of ConA-induced proliferative responses of thymocytes. LHRH and LHRH-immunopositive cells, morphologically similar to thymocytes, were detected in intact fetal thymus. A significant content of LHRH was also found in the peripheral blood of fetuses. The LHRH content in thymus and plasma was similar in males and females. Surgical ablation of the hypothalamus resulted in 2-fold decreases in thymus and plasma levels of LHRH in 21-day-old fetuses compared to sham-operated fetuses. It was concluded that LHRH regulates mitogen-induced proliferative responses of thymocytes during prenatal ontogenesis in the rat. The main source of plasma LHRH at that period is the hypothalamus. Moreover, LHRH is synthesized in the fetal thymus. Thus, LHRH is suggested to have not only a central effect but also to be involved in autocrine or paracrine regulation of proliferative immune responses.