A scald burn injury of 40-60 per cent body surface area was applied to the sodium pentobarbitone anaesthetized rat. The cardiac output of the burn injury rats fell from 39.5 +/- 2.1 to 27.7 +/- 1.5 ml/min (P less than 0.001) while heart rate, mean arterial blood pressure and central venous pressures were little changed but the total peripheral resistance rose significantly. Cardiovascular function was unchanged in control, mock-burned rats. The blood volume was measured continuously using an extracorporeal circuit and was observed to fall significantly by 0.78 +/- 0.36 ml in 5 min (P less than 0.05) and 1.65 +/- 0.38 ml in 60 min (P less than 0.001) after burn injury. The falls in blood volume and cardiac output were virtually simultaneous, and had occurred by 5 min. It is proposed however that this fall in blood volume is not sufficient to cause the observed changes in cardiac output and that additional factors such as cardiac impairment may be responsible for these changes postburn.