Cytochemically evidenced cytochrome oxidase activity was morphometrically measured in the cerebellar cortex of adult and old rats. The ratio (R) between the area of the precipitate due to the cytochemical reaction and the overall area of each mitochondrion was calculated. While in adult rats an inverse correlation between mitochondrial size and R values (r = -.905) was envisaged, in old animals increasing values of R were paired by increases in mitochondrial area (r =.561). Paired-quartile comparisons of the R values from adult and old animals documented a marked age-related impairment of the mitochondrial metabolic competence in small (I quartile: -31.6%) and medium-sized (II quartile: -26.4; III quartile: -16.4) mitochondria, while large organelles showed the lowest age-related decrease (IV quartile: -3.0%). The present findings support that a marked dysfunction of small and medium-sized mitochondria contributes to the significant decay of energy metabolism currently reported in physiological aging.