After hypophysectomy luteinizing hormone /LH/ did not disappear from rat blood serum within 2 months; but concentration of the hormone decreased 2-fold. Administration of luliberine into the hypophysectomized rats led to approximately 3-fold increase of the LH concentration in blood serum within 20 min. The hormones from blood serum of the hypophysectomized rats, from hypophysis and blood serum of intact rats exhibited similarity in their immunochemical and chromatographic properties. The data obtained suggest the presence of extrahypophyseal sources of LN in rats. By means of radioimmunochemical procedure immunoreactive LH was found in 14 structures of the central nervous-system/brain cortex, hypothalamus, hippocampus, medulla oblongata etc/as well as in some peripheral tissues and glands/heart, liver, pancreas etc/. Chromatographically pure LH from liver tissue of the hypophysectomized rats exhibited the biological activity similar to that to the hypophyseal hormone, which was estimated by stimulation of testosterone biosynthesis in the testicular tissue in vitro.