Dendritic spines and synapses were quantitated in the subiculum of 12 Macaca mulatta from 7 to 29 years of age. Dendritic spines were analyzed in six dendritic foci of pyramidal neurons impregnated by the Golgi-Cox method. The dendritic foci examined were the apical shaft, apical oblique, apical tuft, apical terminal, basal terminal, and basal dendrites. Electron microscopic observations were made on synapses stained with ethanolic phosphotungstic acid. There was no difference in spine density between a group of young (7 to 12 years of age) and middle-aged (18 to 21 years) animals. The mean spine density was 1.09/microns for the young and 1.15/microns for the middle-aged animals. In contrast, the spine density of the old animals was 0.87/microns, which was significantly lower than the spine density found in the young and middle-aged animals. A loss of dendritic spines in the old animals occurred equally in all six dendritic foci. A decline in synaptic density in the aged animals were further evidenced by an ultrastructural analysis. The synaptic density of the old animals was 8.3 X 10(8)/mm3, significantly lower than 9.5 X 10(8)/mm3 and 10.2 X 10(8)/mm3 found in the young and middle-aged animals, respectively. This study on synaptic density demonstrated the decline in synaptic number as a function of age in M. mulatta.