Using specific antibodies to corticotrophin-releasing factor (CRF) and arginine vasopressin (AVP) we have immunocytochemically located cells and fibres containing these peptides in sheep fetuses. CRF neurones were first detected, in low frequency, in the paraventricular nuclei of fetuses of 90 days of gestation (term is 142–152 days), although no connections to the median eminence (ME) were apparent until 105 days of gestation. AVP neurones were found from 42 days of gestation, the earliest stage examined, and at this age, fibres and terminals were present mainly in the external lamina of the ME. At subsequent ages there appeared to be a redistribution of AVP fibres such that by 132 days of gestation AVP projections were more or less equally distributed between the lamina of the ME, whereas in the adult AVP fibres are directed mainly to the internal lamina. The early appearance of AVP relative to CRF is consistent with the proposal that AVP may be a significant factor controlling adrenocorticotrophin release in the ovine fetus prior to 105 days of gestatio