In adults, the responses to acute haemorrhage vary greatly depending on the amount of blood lost. While many studies have documented fetal responses to mild haemorrhage, fetal responses to severe haemorrhage are not known. In this study we examined the effect of acute, severe haemorrhage in fetal lambs. Despite the severity of haemorrhage, we found that mean arterial blood pressure was restored within 2 min, and heart rate was restored within 30 min. This restoration of blood pressure and heart rate was facilitated by an increase in peripheral vascular resistance mediated in part by secretion of catecholamines and plasma renin. In addition, about 40% of the shed blood volume was restored within 30 min by fluid from either the fetal interstitium or placenta. The PO2 of umbilical venous blood increased from 33 +/- 9 mmHg to 49 +/- 17 mmHg 2 min post-haemorrhage, and to 47 +/- 15 mmHg 30 min post-haemorrhage. However, this increase was not sufficient to offset the fall in both haemoglobin concentration and umbilical-placental blood flow, so that oxygen delivery decreased from 21.1 +/- 5.5 ml/min per kg to 9.1 +/- 5.2 ml/min per kg 2 min post-haemorrhage, and 14.1 +/- 9.2 ml/min per kg 30 min post-haemorrhage. Because of this decrease in oxygen delivery, oxygen consumption fell and a metabolic acidemia ensued. Nevertheless, oxygen delivery to the heart and brain was maintained because hepatic vasoconstriction diverted more of the well oxygenated umbilical venous return through the ductus venosus. Although the fetus was able to tolerate acute loss of 40% of blood volume, larger volumes of haemorrhage resulted in fetal death.