Effect of melatonin on the gonadotropic activity of the hypothalamo-hypophysial system was investigated. The following proved to occur under the action of melatonin in sexually immature female rats: reduction of the spontaneous activity of luteinizing hormone (LH), of the LH content in the hypophysis, and of the endogenous luliberin stores in the arcuate nucleus; response of the hypophysis to the exogenous luliberin was also diminished. Single administration of low doses (from 1 to 100 micrograms) of melatonin was accompanied by reduction of cAMP content in the hypophysis and the hypothalamus. LH, luliberin and cAMP were determined by radiochemical methods. The results obtained indicated that the inhibitory influence of the pineal gland on the reproductive system was realized by melatonin, its hormone, through the hypothalamo-hypophysial complex.