A novel, long-acting transdermal fentanyl solution (TFS) that delivers sustained plasma fentanyl concentrations following a single application for the control of postoperative pain has recently been approved for use in dogs. The pharmacokinetics (PKs) of this formulation have been evaluated in healthy laboratory dogs, but they have not been reported in a clinical population of dogs for which it is indicated. Plasma fentanyl concentrations were determined from 215 dogs following a single, small-volume (∼50 μL/kg) dose of TFS administered 2-4 h prior to orthopedic or soft tissue surgery. A population PK model was fit, and a 1-compartment open PK model with first-order absorption and an absorption lag-time best described the data. No tested clinical covariates had a significant effect on the PKs. The final model adequately described the population PKs and gave results consistent with laboratory PK studies in healthy dogs. The PKs were primarily characterized by a rapid initial increase in plasma fentanyl concentrations and a long terminal half-life of 74.0 (95% C.I. [54.7-113]) h governed by flip-flop kinetics for the typical subject. The plasma fentanyl concentrations were sustained over days in the range considered to be analgesic for postoperative pain in dogs.