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The proliferative activity, the organization and the corticosterone expression of adrenocortical cells in an experimental adrenal regeneration process after the transplantation of neonatal adrenal glands to adult hosts was investigated. Three days after transplantation, the medullar and the innermost adrenocortical cells of the neonatal adrenal glands showed degenerative and necrotic changes due to the lack of vascular supply. The remaining outermost adrenocortical cells did not display any PCNA immunoreaction. The first PCNA expression, pointing out the beginning of the proliferative cycle, was observed in a 45.4% of the adrenocortical cells, one week after transplantation. After three weeks, several regenerated adrenocortical nodules with a bigger size than the one observed in the previous periods were seen. In these nodules, while the outermost adrenocortical cells were disposed in parallel to the capsule or in rounded groups, the bulk of the regenerated mass width was composed of cells forming longitudinal cords. PCNA immunoreaction was almost exclusively restricted to subcapsular cells (62.5%) and to cells of the outermost portion of the cords (32.5%), the global percentage of PCNA immunopositive cells being 18.4%. Twelve weeks after transplantation, regenerated adrenocortical cells were arranged in three layers: glomerulosa, fasciculata and reticularis. Only 1.85% of the adrenocortical cells were PCNA immunopositive. Although in the early stages of the regeneration process, all the adrenocortical cells, both proliferating and non proliferating cells expressed corticosterone, a restriction of this immunoreactivity to the zonae fasciculata and reticularis was observed when cell zonation was apparent.