BACKGROUND It remains unclear whether analgesia from intraperitoneal local anesthetics is via local or central mechanisms. This double-blind clinical trial tests the hypothesis that intraperitoneal local anesthetic is superior to continuous IV infusion for pain management. Primary outcome was morphine consumption during 0 to 24 h. METHODS Informed consent was obtained from 60 patients, age 30 to 75 yr, American Society of Anesthesiologists physical status I to II, undergoing abdominal hysterectomy. A computer-generated program randomized patients in parallel arms to group IV: continuous infusion of lidocaine 50 mg/h (10 ml) IV and saline 10 ml/h intermittently intraperitoneal; group IP: injection of lidocaine 50 mg/h (10 ml) once every hour intraperitoneally and continuous infusion of saline 10 ml/h intravenously; and group P (placebo): saline 10 ml/h both intravenously and intermittent intraperitoneal injection. Postoperative morphine consumption, pain intensity, recovery, home discharge, and lidocaine concentrations were measured. RESULTS Morphine consumption during 0 to 24 h was lower in group IP versus group IV, mean difference -22.6 mg (95% CI, 11.4 to 33.8; P < 0.01). No difference was seen between group IV and group P. The total mean plasma concentration of lidocaine in group IP was significantly lower than group IV, 0 to 4.5 h postoperatively (P = 0.03) with no evidence of systemic toxicity. Pain intensity and other recovery parameters were similar between the groups. CONCLUSIONS The lower supplemental morphine consumption and plasma lidocaine concentration in group IP would confirm that the effects of local anesthetics are likely to be predominant via local intraperitoneal receptors or anti-inflammatory effects and not via central mechanisms alone.