BACKGROUND The pharmacokinetics and clinical efficacy of ropivacaine (2.5 mg/ml) during a 24-h continuous epidural infusion for postoperative pain relief in 20 patients scheduled for abdominal hysterectomy were characterized using an open-label, increasing-dose design. METHODS Through an epidural catheter inserted at T10-T12, a test dose of 7.5 mg ropivacaine was given 3 min before a bolus dose of 42.5 mg and immediately followed by a 24-h continuous epidural infusion with either 10 or 20 mg/h. Peripheral venous plasma samples were collected up to 48 h after infusion, and urinary excretion was followed up to the end of infusion. Postoperative pain at rest, on coughing, and at mobilization was assessed by means of a visual analog scale 2,4,6,8,12, and 24 h after the end of surgery. Sensory (pinprick) and motor block (modified Bromage scale) were assessed at the same intervals. RESULTS The total plasma concentrations of ropivacaine increased markedly and consistently during the 24-h epidural infusion, in contrast to stable unbound concentrations. Both total and unbound plasma concentrations at the end of infusion were proportional to the total dose, although only the latter was proportional to the infusion rate. The total and unbound plasma clearance was independent of dose. Total mean clearance decreased on average by 21% (P < 0.001) during the last 12 h of epidural infusion, i.e., from 539 +/- 191 ml/min to 418 +/- 138 ml/min, indicating time-dependent kinetics. The unbound clearance also varied between estimates after 8 h of infusion and the end of treatment, i.e., a 5.3% decrease from 10.4 +/- 5.3 l/min to 9.5 +/- 3.9 l/min (P < 0.05). The unbound fraction of ropivacaine in plasma decreased during treatment, and this was related to the increase in alpha1-acid glycoprotein concentration. Pain was generally well controlled, and median visual analog scale scores during mobilization were less than 30 mm in patients receiving ropivacaine at 20 mg/h. CONCLUSIONS The pharmacokinetics of ropivacaine were independent of dose, but total clearance decreased with time over 24 h. The consistent increase in total plasma concentration during the postoperative epidural infusion contrasted to much less variation in the unbound plasma concentrations of ropivacaine.