Glucose and insulin relationship with aging were studied in fed rats. Levels of basal circulating glucose did not change while those of RIA-insulin increa!ed and RIA-glucagon decreased lincary with animal weight. The oral glucose tolerance test revealed a greater increase in blood glucose in adult and old rats than in prepubcrals, while the rise in plasma insulin was faster and greater in the oldest group. After intravenous glucose load, plasma insulin increase was greater in adult than in prepuberal and old rats, and in the latter group values remained elevated for a longer period. The hypoglycemic response to i.v. insulin was greatest in the prepuberals with no difference between adult and old rats. In prepuberals, the augmented insulin sensitivity was counteracted by retarded msulinotropic glucose action and an enhanced basal glucagon level, while in the old animals normoglycemia was maintained due to an augmented secretory response of B cells. counteracted by reduced sensitivity to endogenous insulin.