Pregnancy is characterized by increased plasma ACTH and cortisol. Studies suggest that progesterone acts as an antagonist at mineralocorticoid receptors. Therefore we tested the hypothesis that chronic progesterone, produced by treatment of nonpregnant ewes or during pregnancy, will result in increased plasma ACTH relative to the plasma cortisol concentrations. We studied three groups of ewes: ovariectomized nonpregnant, nonpregnant treated with progesterone, and pregnant ewes. In two series of studies, ewes were adrenalectomized and replaced with 0.35 mg/kg/d cortisol or 0.5 mg/kg/d. In both studies aldosterone was infused at 3µg/kg/d. In the first study, additional infusions of cortisol over 24h were used to increase daily replacement doses to 0.55, 1 or 1.5 mg/kg/d, and intact pregnant and nonpregnant ewes were studied with infusions of cortisol at 0, 0.5 and 1 mg/kg/d. In adrenalectomized ewes chronically replaced to 0.35 mg/kg/d cortisol, plasma ACTH concentrations were significantly decreased in the nonpregnant progesterone-treated ewes as compared to the ovariectomized nonpregnant ewes. With 0.5 mg/kg/d cortisol, plasma ACTH levels were greater in pregnant ewes than in nonpregnant ewes with or without progesterone. Overall plasma ACTH levels at 0.35 mg/kg/d were significantly related to the plasma protein concentration, suggesting that the ACTH levels in the hypocorticoid ewes are most closely related to plasma volume. Across all steroid doses, ACTH was positively related to plasma proteins and progesterone, and negatively related to cortisol. We conclude that increased progesterone does not alter the feedback relation of cortisol to ACTH, but may modulate ACTH indirectly through plasma volume.