The effect of age on the rate of insulin removal from plasma was studied in both rat and man. The experimental approach was based on measurement of the steady-state plasma insulin concentration achieved during a period in which endogenous insulin secretion was suppressed and exogenous insulin infused. Rats, 1½ and 12 mo of age, were infused with 2.5, 5.0, and 10.0 μU/kg of insulin during a 180-min period in which endogenous insulin secretion was suppressed by epinephrine and propranolol. Steady-state plasma insulin concentrations were approximately twice as high in the older rats at every insulin infusion rate. Similar results were seen in man; significant correlations were observed between height of steady-state plasma insulin concentration and advancing age during infusion of exogenous insulin and suppression of endogenous insulin with either exogenous insulin (r = 0.66, P < 0.001) or epinephrine and propranolol (r = 0.47, P < 0.01). Since infusion rates of exogenous insulin were identical in all studies, these results suggest that there is an age-related decrease in insulin catabolism.