Cardiopulmonary parameters were measured in 12 ponies (small horses) before anaesthesia and, following induction with xylazine and ketamine, during maintenance of anaesthesia with desflurane. In six of the ponies (group A) anaesthesia was maintained for three hours with desflurane at an endtidal concentration of 7.4 per cent. In the other six ponies (group B), anaesthesia was maintained in the same way for one hour and then the effects of end-tidal desflurane concentrations of 7.4 per cent and 9.6 per cent with and without artificial ventilation were investigated. In group A ponies the arterial blood pressure and the systemic vascular resistance index (sviu) decreased significantly during the first 45 minutes of anaesthesia but recovered with time. The cardiac index and heart rates were unchanged throughout the measurement period but arterial carbon dioxide tensions increased significantly. In group B ponies, with either mode of ventilation, increasing desflurane concentration resulted in decreases in arterial blood pressure, cardiac index and mixed venous oxygen tension, although the changes were not always statistically significant. There were marked individual differences in the cardiovascular responses to the high desflurane concentrations, the minimum mean arterial blood pressure ranging from 35 to 62 mm Hg, and the cardiac index from 23 to 50 ml/kg/min. The study concludes that during maintenance of anaesthesia with end tidal concentrations of desflurane of 7.4 per cent, cardiac index is well maintained and the initial fall in arterial blood pressures results from a fall in svRi. However, increasing the concentration of desflurane causes a fall in blood pressure due to cardiac depression.