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A longitudinal study of plasma glucose and insulin concentrations in ad libitum fed and dietary restricted male F344 rats was carried out. The life span diurnal pattern of plasma glucose concentration was such that through most of the day dietary restricted rats have significantly lower plasma glucose levels than ad libitum fed rats. Throughout the life span, dietary restricted rats maintain mean 24-hour plasma glucose concentrations about 15% below those of ad libitum fed rats. Plasma insulin levels are maintained in dietary restricted rats at about 50% of the levels in ad libitum fed rats. Although plasma glucose and insulin levels are lower, dietary restricted rats use glucose fuel at the same rate per unit of metabolic mass per day as rats fed ad libitum. While these findings are consistent with the glycation hypothesis of aging and with our hypothesis that dietary restriction retards the aging processes by altering the characteristics of fuel use, they do not establish the validity of either. It is possible that this effect of dietary restriction on carbohydrate metabolism plays no role in its antiaging action. Further studies are required to define the role of these altered characteristics of carbohydrate metabolism in the aging processes.