This study was designed to investigate adrenocorticotropin (ACTH), cortisol, and renin responses to nitroprusside-induced hypotension in adult sheep. Five sheep were surgically prepared with carotid arterial skin loops at least 1 yr before these experiments. After catheterization of the carotid arteries and external jugular veins the sheep were infused with nitroprusside intravenously at rates of 0, 10, 15, or 20 micrograms . kg-1 . min-1 for 10 min. Nitroprusside produced significantly dose-related decreases in mean arterial pressure and increases in heart rate, plasma ACTH and cortisol concentrations, and plasma renin activity. Hematocrit was significantly increased in the 10- and 20-micrograms . kg-1 . min-1 groups during nitroprusside, probably reflecting contraction of the spleen. After the end of the period of hypotension, hematocrit was significantly decreased in all nitroprusside infusion groups, probably reflecting transcapillary movement of fluid into the vascular space. A posteriori analysis of the data suggests that the ACTH response to nitroprusside infusion was better predicted by the nadir in mean arterial pressure and that the renin activity response was better predicted by the initial rate of decrease of mean arterial pressure during nitroprusside infusion.