 This two part study will determine the appropriate dosing regimen of aprepitant for the prevention of postoperative nausea and vomiting PONV in pediatric participants 6 months to 17 years of age, by assessing pharmacokinetic parameters and monitoring safety and tolerability of administered doses. Part I will be an open label investigation of a single dose of aprepitant measuring pharmacokinetics at specified time points up to 48 hours after aprepitant dosing. Part II will be a double blind trial of participants randomized to receive either aprepitant or ondansetron. 