Pituitary-adrenal function was investigated in two groups of Pietrain pigs differing by their susceptibility to the malignant hyperthermia syndrome induced by halothane, and in Large White pigs (LW). Plasma basal levels of glucocorticoids, measured by a protein binding technique, and plasma ACTH concentrations, determined by radioimmunoassay, did not differ according to halothane susceptibility but were higher in LW pigs compared with Pietrain pigs. The reactivity of the pituitary-adrenal axis to such stressors as exposition to a novel environment or shaking was the same for all experimental groups. The same was true concerning plasma levels of glucocorticoids and ACTH after injection of dexamethasone and response of the adrenal cortex to a standard dose of ACTH. These results demonstrate that LW pigs have a higher tonic secretion of ACTH than Pietrain pigs, but with identical phasic response to stimulations. Halothane susceptibility by itself appears to have no influence of pituitary-adrenal function.