In four mature chronically catheterized fetal sheep in utero arterial pressure, heart rate, arterial pH and haematocrit fell during a 16-70% haemorrhage, while Pao2 rose. Plasma vasopressin concentrations increased and were correlated with the percentage of blood volume removed. Following haemorrhage arterial pressure and heart rate were restored within 60 min, while hyperozaemia and acidaemia persisted. Plasma antidiuretic hormone (ADH) concentrations remained the same or increased and were significantly related to the degree of acidaemia. Upon return of the removed blood, pressure rose transiently and Pao2 fell; pH remained low and plasma ADH concentrations fell, but were still related to the degree of acidaemia. In three immature, exteriorized fetuses (0.4 of term) plasma vasopressin concentrations also rose during haemorrhage. The results indicate that fetal plasma vasopressin levels rise during haemorrhage in response both to hypovolaemia and the subsequent acidaemia. Further the response to haemorrhage is present at an early gestational age.