Studies were performed to test the hypothesis that the absence of adrenal glucocorticoids late in gestation alters sympathetic and baroreflex responses before and immediately after birth. Fetal sheep at 130-131 d gestation (term 145 d) were subjected to bilateral adrenalectomy before the normal prepartum increase in plasma cortisol levels. One group of fetuses (n=5) received physiologic cortisol replacement with a continuous infusion of hydrocortisone (2 mg/d/kg for 10 d), the other group received 0.9% NaCl vehicle (n = 5). All animals underwent a second surgery 48 h prior to study for placement of a renal nerve recording electrode. Heart rate (HR), mean arterial blood pressure (MABP), renal sympathetic nerve activity (RSNA) and baroreflex control of HR and RSNA were studied before and after cesarean section delivery. At the time of study (140-141 d gestation) fetal plasma cortisol concentration was undetectable in adrenalectomized (ADX) fetuses and 58 ± 9 ng/ml in animals receiving cortisol replacement (ADX + F). Fetal and newborn MABP was significantly greater in ADX +F relative to ADX animals. One hour after delivery MABP increased 13 ± 3 mmHg and RSNA increased 91 ± 12 % above fetal values in ADX +F (both p < 0.05) but remained unchanged in ADX lambs. The midpoint pressures of the fetal HR and RSNA baroreflex function curves were significantly greater in ADX + F (54 ± 3 and 56 ± 3 mm Hg for HR and RSNA curves, respectively) than ADX fetuses (45 ± 2 and 46 ± 3 mm Hg). After delivery, the baroreflex curves reset toward higher pressure in ADX+F but not ADX lambs. These results suggest that adrenal glucocorticoids contribute to cardiovascular regulation in the late gestation fetus and newborn by modulating arterial baroreflex function and sympathetic activity.