The minimum alveolar concentration of desflurane when combined with a continuous infusion of medetomidine at 3.5 microg/kg/hour was measured in seven ponies. Anaesthesia was induced with medetomidine (7 microg/kg intravenously) followed by ketamine (2 mg/kg intravenously) and maintained with desflurane in oxygen. The infusion of medetomidine was started 20 minutes after the induction of anaesthesia. The electrical test stimulus was applied at the coronary band (50 V, 10 ms bursts at 5 Hz for one minute), and heart rates and rhythms, arterial blood pressures, and arterial blood gas tensions were measured at intervals, just before the application of the stimulus. The mean (sd) minimum alveolar concentration of desflurane was 5.3 (1.04) per cent (range 3.2 to 6.4 per cent), 28 per cent less than the previously published value for desflurane alone after the induction of anaesthesia with xylazine and ketamine. The cardiopulmonary parameters remained stable throughout the period of anaesthesia. The mean (sd) time taken by the ponies to stand after the administration of desflurane ceased was 16.5 (6.17) (range 5.8 to 26) minutes, and the quality of recovery was good or excellent. However, one pony died shortly after standing; a postmortem examination revealed that it had chronic left atrial dilatation.