Quantitative investigation of GABA-immunopositive and GABA-immunonegative neurons was performed in grafts of embryonic tissue of somatosensory cortex, (1) injected intraparenchymally into the barrel field of adult rats (n = 5); (2) placed in an acute cavity in the same area (n = 9); or (3) implanted into the anterior eye chamber (n = 5). Analysis of initial embryonic tissue at the age of grafting (E17) demonstrated high numerical density of neurons both GABA-positive and other. Six months after grafting the total neuronal density was higher than in normal cortex in both groups of intracortical grafts, but was lower than normal in the intraocular grafts. The numerical density of GABA-positive somata, however, decreased in all types of the grafts, and their ratio to GABA-negative neurons was significantly lower than in normal neocortical tissue. The total numerical density of the neurons in the grafts had strong positive correlation to the degree of morphological graft/host integration, which was evaluated as a ratio of extent of area of direct graft/host neuropil confluence versus the total extent of the border. The mean diameters of nuclei of GABA-negative cells in both groups of intracortical grafts did not differ from those of the intact neocortex, but in the isolated grafts they were substantially larger. Nuclei and somata of the GABA-positive cells, however, were hypertrophic in all groups of grafts, and especially in the intraocular ones. Measurements of the diameters of host neocortical neurons adjacent to the grafts also exhibited a less marked but still significant hypertrophy. The possible role of various factors (synaptic, trophic, functional), determining the described alterations in neuronal populations within the grafts is discussed.