Neocortical tissue obtained from the 8-day old rat embryos was transplanted into the cerebellum of 10-day-old rats. Within these transplants various subregions were seen, some with laminated cytoarchitecture and others with nuclear organization. The laminated structures were found to contain the pyramidal nerve cells of the neocortex. The differentiation of dendrites of these pyramidal cells was studied in Golgi-Cox preparations. The sequence of dendritic growth and differentiation and the pattern of dendritic arborization were found to be very similar to those seen in the pyramidal cells of the neocortex. These two attributes of dendritic differentiation of the pyramidal cells were therefore considered to be predetermined. However, the course and the length of the apical dendrites were found to be greatly altered; therefore, these two characteristics were evaluated and found to be influenced by the milieu immediately surrounding the transplanted neuronal elements. Futhermore, the apical dendrites were found to form bundles. It was not possible to establish whether or not this characteristic of dendritic organization was predetermined. Electron microscopic evaluation revealed the presence of axodendritic and axosomatic synapses on the pyramidal cells. These observations suggested that the transplanted neuronal elements may be physiologically active, and that if trophic influences mediated by the synaptic endings had contributed to the differentiation of the pyramidal cells and their dendrites, they could not have been specific in nature.