The pharmacokinetics of sufentanil, a new thienyl analogue of fentanyl, were studied in 10 surgical patients. Sufentanil, 5 micrograms/kg, was given intravenously as a bolus injection and plasma concentrations measured at intervals up to 8 h. Plasma sufentanil concentrations decreased rapidly after injection--98% of the administered dose having left the plasma within 30 min. In 9 of the 10 patients, a tri-exponential equation optimally described the sufentanil concentration decay curve, with average (+/-SEM) half-lives for the rapid (pi) and slow (alpha) distribution phases of 1.4 +/- 0.3 min and 17.7 +/- 2.6 min, respectively. The average terminal elimination (beta) half-life was 164 +/- 22 min. The average value for Vd beta was 2.9 +/- 0.2 1/kg, Vdss 1.7 +/- 0.2 1/kg and total plasma clearance 12.7 +/- 0.8 ml X kg-1 X min-1 (935 +/- 50 ml/min). In one patient, a bi-exponential equation was sufficient to describe the concentration-time data, yielding a distribution half-life of 4.7 min and an elimination half-life of 117 min.