Ten pairs of male Long-Evans rats living in nonenriched environments (3 rats per small cage) were transferred to either enriched environments (10 rats per large cage plus "toys") or nonenriched environments (2 rats per small cage) at 766 days of age. One hundred and thirty-eight days later, at 904 days of age, the cerebral cortical thickness from these animals was measured on projected, 10-micron, thionine-stained, transverse sections. Although the thickness in the enriched rats was greater than in the nonenriched rats in all sections through the frontal, parietal, and occipital cortices, the 4 to 10% differences were statistically significantly different in only the frontal and occipital cortices. Right greater than left cortical thickness differences were not statistically significant in either the enriched or the nonenriched animals by 904 days of age. Neuron and glial counts were made on enlarged photographs of area 18 in the occipital cortex on 6-micron-thick, luxol fast blue-stained sections. No significant differences in cell counts were noted between the enriched and nonenriched animals. No significant differences in neuronal counts were found among 108-, 650- (from previous experiments), and the 904-day-old nonenriched rats. The notable findings were the plasticity of the extremely old, enriched rats' occipital cortex and the lack of the loss of neurons in cerebral cortical area 18, whether or not the environments were enriched. These results showed that the cerebral cortex remained structurally plastic throughout the lifetime of the organism.