The influence of caloric restriction (CR) initiated at 17 months of age was investigated on selected age-associated measures in skeletal muscle. Tissue from young (3-4 months) ad libitum-fed, old (30-32 months) restricted (35% and 50% CR, designated CR35 and CR50, respectively), and old ad libitum-fed rats (29 months) was studied. CR preserved fiber number and fiber type composition in the vastus lateralis muscle of the CR50 rats. In the old rats from all groups, individual fibers were found with either no detectable cytochrome c oxidase activity (COX-), hyperreactivity for succinate dehydrogenase activity (SDH++; also known as ragged red fibers [RRF]), or both COX- and SDH++. Muscle from the CR50 rats contained significantly fewer COX- and SDH++ fibers than did the muscle from CR35 rats. CR50 rats also had significantly lower numbers of mtDNA deletion products in two (adductor longus and soleus) of the four muscles examined compared to CR35 rats. These data indicate that CR begun in late middle age can retard age-associated fiber loss and fiber type changes, as well as increases in the number of skeletal muscle fibers showing mitochondrial enzyme abnormalities. CR also decreased the accumulation of mtDNA deletions.