To determine the role of endogenous ACTH and hyperventilation in the adrenocortical response to hypoxia, pentobarbital-anesthetized dogs equipped with left adrenal venous cannulas for measurement of cortisol secretion rate (CSR) and arterial cannulas for measurement of plasma ACTH were exposed to 20 min of normoxia (group I), spontaneous ventilation, normocapnic hypoxic hypoxia (group II), controlled ventilation, normocapnic hypoxic hypoxia (group III), controlled ventilation, normocapnic carbon monoxide hypoxia (group IV), or hypoxic hypoxia with elevated carboxyhemoglobin (group V). Group I showed no change in ACTH and CSR. Groups II and III greatly increased CSR whereas only group III increased ACTH significantly. Group IV greatly increased ACTH whereas CSR increased but less than group III. Group V showed a significant increase in ACTH but no significant CSR response. In addition, 5 U of ACTh were infused in several animals from groups I, III, and IV. Exogenous ACTH caused increases in CSR that were larger in group I than groups III and IV. The data are consistent with the hypothesis that ACTH in arterial blood is not the sole controller of CSR during hypoxic stress.