The pharmacokinetics of thiopentone sodium administered intravenously as a single dose (11 mg/kg) were studied in acepromazine pre-medicated horses and ponies in which anaesthesia was maintained with either halothane (Group 1) or isoflurane (Group 2). The results showed that the disposition kinetics of thiopentone in horses and ponies were best described by a three-compartment open model. In plasma, a very short initial distribution phase in both horses and ponies, half-life 1.4 +/- 1.2 min (mean +/- SD) and 1.3 +/- 0.7 min, respectively, was obtained, which was followed by a second comparatively slower redistribution phase, half-life 16 +/- 12 min and 11 +/- 5 min, respectively. The volume of distribution for the drug was large, especially in the ponies which received isoflurane (1127 +/- 86 ml/kg), compared to the horses which received halothane (742 +/- 89 ml/kg). The drug had a somewhat shorter elimination half-life in the horses (147 +/- 21 min) than in than ponies (222 +/- 44 min), but no obvious difference in clearance of the drug was observed between the horses (3.5 +/- 0.5 ml/min/kg) and ponies (3.6 +/- 0.8 ml/min/kg).