Studies were performed to determine whether the isolated ovine anterior and intermediate pituitary might rhythmically secrete three POMC peptides, ACTH, ir-β-endorphin (ir-β-EP), and ir-α-melanocyte stimulating hormone (ir-α- MSH) in vivo. When blood was taken at 10-min intervals from four ewes with hypothalamo-pituitary-disconnection (HPD), a distinct POMC-peptide and cortisol ultradian rhythm was noted. A comparison of the four HPD ewes with five nonstressed hypothalamopituitary-intact (HPI) ewes revealed that the mean plasma levels of the three POMC-peptides and cortisol were increased, the mean ACTH and ir-α-MSH pulse amplitudes were increased, and the mean ir-β-EP and ir-α-MSH interpulse intervals were decreased.When four HPI ewes were subjected to a mild stress, plasma POMC-peptide and cortisol levels increased significantly when compared with the five unstressed HPI animals. In addition, the ACTH and cortisol pulse amplitudes increased and the ir-β-EP and ir-α-MSH interpulse intervals decreased. Although plasma ACTH levels in the stressed HPI and HPD ewes were comparable, mean plasma cortisol levels were 2-fold greater in the stressed HPI animals.To determine whether the ACTH hypersecretion in the HPD ewe might reflect a net reduction in hypothalamic inhibitory influence over ACTH secretion, we examined the effects of dopamine (DA), somatostatin (SS-14), and rat atrial natriuretic peptide [rANF(l-28)] on the secretion of ACTH from cultured ovine anterior pituitary cells. DA and SS-14 did not exert a discernible effect on basal, CRF-, or arginine vasopressin (AVP)- stimulated ACTH secretion. Although basal ACTH secretion was unaffected by rANF(l-28) (10-12-10-8 M), a significant inhibition of CRF- and AVP-stimulated ACTH release was observed. Furthermore, when 100 µg rANF(l-28) was injected via the external carotid artery into four HPD ewes, the rise in plasma ACTH and cortisol did not differ from that seen in four HPD ewes injected with NaCl.We conclude that: 1) pulsatile POMC-peptide secretion is an intrinsic property of the corticotrope and melanotrope; 2) POMC-peptide secretion in the HPI ewe represents the modification of this intrinsic pituitary rhythm by the hypothalamus; 3) in the HPI ewe, activation of the hypothalamus by mild stress increases plasma ACTH to levels seen in the HPD ewe, and increases both the ACTH pulse frequency and pulse amplitude; 4) the increased plasma levels of ir-β-EP and ir-α-MSH and decreased ir-β-EP interpulse interval after HPD suggests that the neural input to the intermediate lobe restrains intermediate lobe POMC-peptide secretion; 5) the increased plasma ACTH and cortisol levels after HPD suggests that the hypothalamus also exerts a tonic inhibitory influence over POMC peptide synthesis and/or secretion by the anterior pituitary; however, this putative inhibitory influence is not mediated by dopamine or SS-14; 6) the ability of ANF(l-28) to inhibit CRF- or AVPmediated ACTH secretion in vitro suggests that the ovine corticotrope may possess intracellular mechanisms that inhibit ACTH secretion which are distinct from those which mediate the feedback actions of the glucocorticoids.