Nuffield Institute for Medical Research, University of Oxford, Oxford, 0X3 9DS (Received 11 March 1976) Vasopressin is considered an important hormone controlling adrenocorticotrophin (ACTH) secretion under some conditions (De Wied, Bohus, Ernst, de Jong, Nieuwenhuizen, Pieper & Yasumura, 1968). Its infusion into dogs at pharmacological but not physiological doses increases plasma ACTH concentration (Andersen & Egdahl, 1964). It behaves as a corticotrophin-releasing factor on incubation with anterior pituitary cells (Portanova, Smith & Sayers, 1970). Moreover a neuroendocrine vasopressin pathway possibly associated with ACTH secretion has been proposed (Parry & Livett, 1973). During hypoxaemia in the foetal sheep there is a rise in both plasma ACTH and vasopressin (Alexander, Britton, Forsling, Nixon & Ratcliffe, 1973; Boddy, Jones, Mantell, Ratcliffe & Robinson, 1974b; Rurak, 1976). Thus the possibility that a rise in foetal plasma vasopressin is responsible for the increased secretion of ACTH during hypoxaemia has been investigated. Foetal and maternal vascular