Subjects will receive intravenous methadone or morphine ("study drug") delivered by an initial IV "bolus" injection followed by a nurse-administered patient controlled analgesia (PCA) device for postoperative pain for a period of 24 hours. The initial dose of study drug will be 0.2 mg/kg IV administered following admission to the ICU after surgery. The study drug will then be given at a dose of 0.035 mg/kg IV as needed q30min via PCA. The study drug may be increased or decreased in increments of 20-25% according to the discretion of the investigator as needed to maintain a FLACC pain assessment tool < 4. Subjects will also receive lorazepam 0.025 mg/kg IV q2hr as needed for agitation as indicated by specific criteria. The study drug will be discontinued after 24 hours to facilitate "wash out" sampling and determination of elimination half-life. Beginning at 24 hours, fentanyl will be used for analgesia at an equianalgesic dose to be determined by the investigator based upon the current PCA "study drug" dose.