This study evaluated the effects of aging and endurance training on the metabolic responses of trained and sedentary young (age 20-32 yr) and older (age 60-70 yr) men to exercise at the same relative exercise stress (70% of maximal O2 consumption). Plasma growth hormone concentrations at rest were similar in all four groups, but both older groups had an attenuated response to exercise. The older trained men appeared to have avoided the age-associated changes that were evident in their sedentary peers with respect to resting plasma insulin, C-peptide, and norepinephrine concentrations. Plasma glucagon concentrations were lower in both older subject groups at rest. Both sedentary groups decreased their plasma glucose concentrations and increased their plasma glucagon concentrations during exercise, whereas the trained groups had increases in their plasma glucose concentrations but had no change in their glucagon concentrations. Thus, although the concentrations of some hormones at rest and during submaximal exercise are unaffected by aging or by training, others are markedly altered by aging, training, or the interaction of the two. However, it appears that older healthy sedentary men undergo less physiological stress than young untrained men during submaximal exercise at the same relative exercise intensity, and they have no responses that would contraindicate their participation in exercise of the duration and intensity usually prescribed in exercise-training programs.