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The work studied effect of serotonin on development of the system of luteinizing hormone-realizing hormone (LHRH) in rats at the prenatal period. LHRH-neurons located along its migration pathway were revealed immunocytochemically at the 18th day of embryonal development (E18 and so on) after serotonin synthesis inhibition by daily intraperitoneal injections of n-chlorophenylalanine from E11 to E17 (experimental group). In control animal group, the saline was injected. The percent content of LHRH-neurons located in the forebrain rostral part was higher in the experimental group as compared to control. The opposite quantitative ratio of neurons was observed in the more caudal (hypothalamic) area. This indicates deceleration of the LHRH-neuronal migration under conditions of serotonin deficit. Study of the LHRH-neuron differentiation showed that in the experimental group the neuron optical density correlating with the LHRH content in the cell decreased as compared to the control group both in males and in females. The size of LHRH-neurons on n-chlorophenylalanine administration decreased, but only in females. Thus, serotonin affects both migration and differentiation of LHRH-neurons.