By means of morphological methods, a study was made of effects of cyclosporin A (Cs A) on developmental dynamics in 15 day old embryo neocortex transplanted into the rat brain. It is shown that Cs A exerts no cytotoxic or cytostatic effects on grafting cells. Differentiation of grafting neuroepithelial cells into neuroblasts, young neurons and neurons in transplants of Cs A-treated rats and control transplants took place at the same time. Mitotic activity of grafting neuroepithelial cells was lower than in developing neocortex in situ; pathological mitotic forms have been described.