It was previously shown that rank-related differences in adrenocortical function existed among the males of a troop of olive baboons living in their natural habitat in East Africa. High-ranking males, by the criterion of reproductive activity, had significantly lower cortisol titers than those of subordinates, when measured immediately after immobilization with anesthetic. However, high-ranking males elevated titers rapidly during the early period of immobilization stress, attaining titers equal to those of subordinates at 60 min. The present study replicates these rank differences and provides possible endocrine mechanisms underlying them. Males with low basal titers (predominantly high-ranking males) had the fastest and most extreme suppressions of circulating cortisol titers after dexamethasone administration. However, they had cortisol clearance rates similar to those of individuals with high initial cortisol titers. These data suggest that the rank-related variance in basal cortisol levels is attributable to differential sensitivity to negative feedback regulation. Males who elevated cortisol titer most rapidly during the early poststress period (again, predominantly high-ranking males) had adrenals no more responsive to an ACTH challenge than those of remaining subjects. This suggests that a rapid adrenocortical secretory response during stress is not attributable to enhanced adrenal sensitivity to ACTH, but rather to an accelerated secretion of ACTH by the pituitary gland.