The authors determined the pharmacokinetics of fentanyl 100 μ·kg−1iv in patients undergoing elective abdominal aortic surgery. The mean (±SD) age of the ten patients was 67.2 ± 8.7 yr; their mean weight was 78.5 ± 13.7 kg. Seven patients had aortic aneurysm repair, and the other three patients had aortobifemoral grafts. Serum fentanyl concentrations were determined from samples drawn at increasing intervals over a 24-h period. A three-compartment pharmacokinetic model was fit to the concentrationversustime data. Total drug clearance was 9.8 ± 1.8 ml · min−1· kg−1. The volume of distribution at steady-state (Vdss) was 5.4 ± 1.9·1 kg−1. Elimination half-time was 8.7 ± 2.5 h. There were no significant correlations between these pharmacokinetic parameters and patient's age, duration of aortic cross-clamping, duration of surgery, intraoperative blood loss, or volume of iv fluids given intraoperatively. In healthy volunteers or patients undergoing general surgery, other investigators report mean elimination half-times for fentanyl ranging from 1.7 to 4.4 h. The prolonged elimination half-time in patients having abdominal aortic surgery has important clinical implications. In particular, recovery from large doses will take much longer than would have been anticipated from previously published fentanyl pharmacokinetic data.