The effect of chronic stress on adrenocorticalfunction was tested in young and aged male and female rats. The rats were subjected twice a day to 2 h restraint stress for 20 days. Adrenocortical responsiveness to stressors was decreased during the treatment in all groups. Responsiveness to ACTH injection was greater than stress responsiveness in all groups. The decrease in stress responsiveness was greater in the young than in the aged groups. The data suggest that elevated corticosterone concentrations from chronic stress adrenocortical activation result in incomplete feedback inhibition of the adrenocortical control mechanism. The increased inhibition in the young groups supports the concept of decreased adrenocortical control system sensitivity in the aged animal.