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Quantitative analysis of the total number and distribution of dendritic spines along the apical shafts of layer V cerebral cortical pyramids has been performed on aging rats (90-120 to 1,135 days old) and on rats during the period of early and late development (10-80 days). As expected from previous work, present results show that the total number of dendritic spines along the shafts increase from 10 to 80 days, after which it starts to gradually decrease until the last age studied (1,135 days). The quantitative analysis of the effect of aging on the relative decrease of dendritic spines shows that this decrease starts being homogeneous along the whole length of the apical shafts and that from a certain age onwards, estimated according to present results in 400 days, this effect is significantly more pronounced in layers IV and III-II than in deep layers. Furthermore, the comparison made between the distribution of dendritic spines along the apical shafts of pyramidal neurons of old and young animals has shown that aging produces a regression of this distribution to juvenile conditions.