The pharmacokinetics of propofol and ketamine administered together by infusion were investigated in four ponies. Blood propofol and plasma ketamine and norketamine concentrations were measured by high performance liquid chromatography. After premedication with detomidine (20 micrograms kg-1) anaesthesia was induced with ketamine (2.2 mg kg-1 intravenously). The trachea was intubated and the ponies were allowed to breathe 100 per cent oxygen. A bolus dose of propofol (0.5 mg kg-1) was then administered intravenously and propofol and ketamine were infused for 60 and 45 minutes, respectively. The average mean infusion rate of propofol was 0.136 mg kg-1 min-1, and the ketamine infusion rate was maintained at 50 micrograms kg-1 min-1. The mean (SD) elimination half-lives of propofol and ketamine were 69.0 (8.0) and 89.8 (26.7) minutes, the mean volumes of distribution at steady state were 0.894 (0.161) litre kg-1 and 1.432 (0.324) litre kg-1; the mean body clearances were 33.1 (4.5) and 23.9 (3.8) ml kg-1 min-1 and the mean residence times for the infusion were 87.1 (4.1) and 110.7 (8.2) minutes, respectively. Norketamine, the main metabolite of ketamine, was detected throughout the sampling period. The mean residence time for norketamine was 144 (16) minutes. All the ponies recovered quickly from the anaesthesia; the mean times to sternal recumbency and standing were 11.1 (5.3) and 30.0 (20.8) minutes, respectively, from the end of the infusion.