The effect of intracerebroventricular infusion of compound 48/80 (C48/80), a mast-cell secretagogue, on adrenal cortisol secretion was investigated in dogs under pentobarbital anesthesia. A marked increase in adrenal cortisol secretion was elicited by C48/80 along with a concomitant increase in the plasma levels of cortisol and immunoreactive-ACTH, but neither arterial blood pressure and heart rate nor the plasma histamine level altered significantly. Pretreatment with either anti-corticotropin releasing factor (CRF) antiserum or pyrilamine maleate (H₁ histamine-receptor antagonist) significantly attenuated the C48/80-evoked increase in cortisol secretion, but pretreatment with metiamide (H₂-receptor antagonist) significantly potentiated it. Significant attenuation of the C48/80-evoked increase in cortisol also occurred in dogs given ketotifen, a mast-cell stabilizing drug, prior to the pharmacologic challenge. In the pars tuberalis (PT) and median eminence (ME), mast cells were highly concentrated in close association with the primary plexus of the hypophyseal portal system. Degranulated mast cells were extensively found in the ME of C48/80-treated animals. These results suggest that mast cells located in these regions liberated histamine within the brain as a result of degranulation induced by C48/80, and that this led to activation of the hypothalamic-pituitary-adrenocortical axis.