The effects of anaesthesia and surgery on the pharmacokinetics of ketobemidone were studied in 12 patients. Plasma ketobemidone concentrations were assayed with a mass-fragmentographic method. The peroperative Vd(area) was 5.9 +/- 2.6L/kg and the terminal half-life was 3.9 +/- 1.7 h. In the postoperative period Vd(area) decreased to 3.7 +/- 0.4L/kg and the terminal half-life to 2.1 +/- 0.4 h. Plasma clearance (Clp) did not change significantly, peroperative Clp being 18 +/- 4.3 ml/min/kg and postoperative Clp being 22 +/- 7.5 ml/min/kg. The pharmacokinetics of ketobemidone were not influenced by the addition of a spasmolytic agent in the commercial combination ketobemidone preparation 'Ketogin'. Postoperative pain was relieved in 15 patients by patient-controlled intravenous administration of ketobemidone by means of a programmable drug injector. The mean ketobemidone consumption was 2.3 +/- 0.8 mg/h, which produced a mean plasma concentration of 28 +/- 11 mg/ml. Pseudosteady-state plasma concentrations of ketobemidone were established with a mean minimum effective concentration (MEC) of 25 +/- 11 ng/ml. Ketobemidone 'plain' and 'Ketogin' did not differ significantly in these respects. Analgesia was considered by all patients to be satisfactory.