A comparative study has been performed of dynamics of development of ectopic transplants of embryonal (E14) neocortex anlages obtained from intact rats and from the rats administered with serotonin inhibitor para-chlorophenylalanine (PCPA) at the 11th day of pregnancy as well as after incubation of such anlages in the serotonin-containing medium. The goal of this work was to elucidate effect of serotonin on division and differentiation of embryonal neocortex cells. Study of degeneration, mitotic activity, and differentiation (by using immunohistochemical detection of nerve cell nuclear protein--NeuN) of transplanted cells has allowed establishing that serotonin promotes survival and differentiation of transplant neuroepithelial cells as well as participates in regulation of their proliferation. It is suggested that serotonin produces stimulation effect on the cell cycle rate of transplantated cells to thereby accelerate neuronal differentiation.