Plasma glucose and insulin responses to a standard oral glucose challenge and a mixed meal were determined for two groups of male volunteers (office workers and laborers) and a group of female housewives or office workers. Although glucose tolerance declined with age to a certain degree in all three groups, the age-related change varied as a function of both level of habitual physical activity and gender. Specifically, the decline in glucose tolerance was greatest in the male office workers and least in the females. The plasma insulin responses did not increase with age in any of the groups. These results suggest that glucose tolerance decreases with age because there is a decline in insulin action, which is not compensated for by an increase in insulin secretion. Insulin sensitivity appears to be enhanced in females as compared with males. Sensitivity is also enhanced in males habitually engaged in physical labor; thereby accounting for the age-related decline being greatest in the male office workers. Finally, the results showed that although the loss of glucose tolerance with age varied from group to group, the quantitative nature of the change was modest in all three groups. These data further emphasize that very little change in glucose tolerance is associated with aging in generally healthy, nonobese individuals.