Subjects will be excluded from the study because of prematurity (gestational age < 37 weeks), weight < 3.0 kg, severe hepatic dysfunction (elevation of conjugated bilirubin > 2, AST/ALT > 200 IU/L), significant renal dysfunction (serum Cr > 1.5 mg/dL), participation within 30 days of study entry or within 5 times the half-life, whichever is longer, in another investigational drug study, or previous participation in this study. Patients will be excluded if they have received opioids for more than 12 hours of the 48-hour period prior to surgery. In addition, patients will be excluded if they have a history of other clinically significant medical problems, which, in the opinion of the investigator, would interfere with study participation.