The pharmacokinetics of absorption and distribution of a single bolus dose of sufentanil 150 micrograms for major abdominal surgery were compared in 20 patients after random intravenous or epidural administration. Samples of plasma and cerebrospinal fluid were taken at regular intervals from time zero to 180 min after injection and at the time of tracheal extubation (3.43 to 12.66h). Sufentanil was analysed by radioimmunoassay. The area under the concentration-time curve (AUC) from zero to 1h, 2h, 3h, tracheal extubation and infinity, the absorption and distribution half-lives, maximum plasma and CSF concentrations, time to the peak concentration of sufentanil, and the fraction of sufentanil that reached the central circulation after epidural administration were assessed. Except in the first sample, plasma concentrations of sufentanil were comparable between the 2 groups. The initial transfer of sufentanil from the epidural space to the systemic circulation appeared to be very rapid. Explanations for this phenomenon are given. In only 3 patients could an uptake of sufentanil from the systemic circulation into the CSF be demonstrated. The transfer of sufentanil from the epidural space into the CSF is slower than the transfer into the plasma and it varied interindividually.