The effects on intramuscular blood flow and cardiopulmonary parameters of changing from anaesthesia with halothane to isoflurane and vice versa were investigated in six ponies (small horses). Anaesthesia was induced with xylazine, ketamine and diazepam, maintained for one hour with halothane at an end tidal concentration of 1 per cent and then with isoflurane at 1.5 per cent for a further hour (halo/iso). On another occasion the order in which the volatile agents were administered was reversed (iso/halo). After one hour of anaesthesia the mean (sd) arterial blood pressure (MAP) and cardiac output (co) of the ponies on the two occasions did not differ significantly (iso/halo, MAP 43  mmHg, co 10.9 [2.4] litre/min; halo/iso, MAP 53  mmHg, co 8.9 [2.3] litre/min). On changing the anaesthetic, MAP rose similarly in both groups. In the halo/iso group co remained stable (8.64 [1.4] litre/min after the hour of isoflurane), but in the iso/halo group, co decreased significantly on the administration of halothane (6.16 [1.3] litre/min after the second hour). When halothane replaced isoflurane, the intramuscular blood flow in both the upper and lower triceps brachii decreased significantly by 23 to 35 per cent, but when isoflurane replaced halothane the changes were not significant. It is concluded that co and intramuscular blood flow both deteriorated when isoflurane was replaced by halothane. When isoflurane replaced halothane, cardiopulmonary function did not deteriorate further, but any improvement was not statistically significant.