Background: The potential for ondansetron to cause ECG changes as QT prolongation is well-reported. Objectives: The aim of this study was to evaluate the effect of parenteral ondansetron on the PR interval, QRS duration and QTc interval of ECG at peak effect and at post peak effect in pediatric patients with vomiting in PED. Patients and methods: The study was conducted in the Pediatric Emergency Department, Sohag University Hospital. ECG was done for 110 patients before and after injection of ondansetron 0.15 mg/ kg for vomiting. 60 Patients received ondansetron by intravenous injection and 50 patients received ondansetron by intramuscular injection. Results: IV ondansetron administration resulted in a significant increase in QTc interval after 15 minutes and 45 minutes p= 0.03 and p= 0.0003 respectively, significant decrease in PR interval after 15 minutes p= 0.02 and there was a significant decrease in QRS duration at 15 minutes after injection of ondansetron p= 0.02. IM ondansetron administration resulted in a significant increase in QTc interval after 30 minutes and 60 minutes p= 0.04, p= 0.0001 respectively, significant decrease in PR interval after 30 minutes and 60 minutes p < 0.0001, p= 0.04 respectively and a statistically significant decrease between QRS duration before and after 30 minutes of ondansetron injection p < 0.0001. Conclusion: It could be concluded that significant ECG changes occurred in children receiving a single dose of parenteral Ondansetron 0.15 mg/ kg. None of the patients had an ondansetron related cardiac adverse events.