Abstract: Synaptic mitochondria, cytochemically positive to cytochrome oxidase (COX) activity, were investigated by morphometric methods in the hippocampal dentate gyrus of adult and old rats. The number of mitochondria/μm3 of tissue (Nv), the volume fraction occupied by mitochondria/μm3 of tissue (Vv), the average mitochondrial volume (V), the longer mitochondrial diameter (Fmax), and the ratio R:mitochondrial area/overall area of the cytochemical precipitate due to COX activity were measured on COX-positive organelles. In old animals, Nv, Vv, V, and Fmax increased at a not significant extent; R was not significantly decreased. The complement (%) of longer organelles was higher in old animals. COX activity is currently considered an endogenous marker of neuronal oxidative metabolism; thus, although our findings refer to the discrete subpopulation of COX-positive organelles located at synaptic terminals, they support that changes of mitochondrial ultrastructure and metabolic competence may contribute to the age-related alterations of neuronal performances.