Senescent hamsters display a marked reduction in volume of voluntary running. The purpose of this study was to determine whether age differences exist in the pattern of fuel utilization during submaximal exercise, which may account for the reduction in voluntary running. Further, we determined the effects of age on muscle oxidative capacity to assess its relationship to endurance performance in senescent hamsters. Depletion of carbohydrate and lipid content of skeletal muscle and liver, and changes in blood concentration of various hormones and substrates during one hour of exercise at 60 percent of VO2 max served to assess age effects on utilization of metabolic substrates. Exercise produced equivalent depletion of muscle glycogen and similar rise in plasma free fatty acids in young and old hamsters. No exercise effects on skeletal muscle triglyceride concentration or on plasma glycerol, glucagon or catecholamine concentrations were noted. With palmitoyl carnitine as substrate (but not with pyruvate) State 3 respiration of cardiac and skeletal muscle homogenates was lower in old compared to young hamsters. Although old hamsters have a reduced capacity to oxidize lipids in vitro, few age differences in fuel use are evident in vivo during submaximal exercise. Thus, these minor age differences in substrate utilization do not likely account for the substantial reduction in the levels of spontaneous running in senescent hamsters.