The pharmacokinetics of alfentanil were investigated in the horse. Four doses of alfentanil (4, 10, 20 and 40 μg/kg) were given to four horses at different times and their locomotor activity monitored. Doses of 20 and 40 μg/kg produced a significant increase in locomotor activity. The plasma concentrations of alfentanil were measured in six standing horses and the pharmacokinetics calculated. It was found that the decay curves were best described by a bi-exponential equation. The elimination half-life (t1/2ß) was 21.65 ± 3.99 min and the clearance (CI) was 14.1 ± 0.7 ml/kg/min. The same horses were anaesthetized with xylazine-ketamine and maintained with halothane in oxygen for the first experiment and isoflurane in oxygen for the second experiment. The pharmacokinetics were again calculated from measured plasma alfentanil concentrations. There were significant differences between the kinetics in the conscious and the anaesthetized animals but there were no significant differences in alfentanil kinetics between the two anaesthetic agents. The t1/2 for alfentanil under halothane and isoflurane anaesthesia were 55.95 ± 20.77 and 68.03 ± 23.22 min, respectively, and the CI values were 14 ± 1.7 and 13.6 ± 1.32 ml/kg/min.