This double blinded, placebo controlled, randomized, and prospective study investigated the effect of the rapid intravenous administration of ondansetron 0.15 mg.kg-1 or metoclopramide 0.25 mg.kg-1 on the heart rate, haemoglobin saturation, systolic blood pressure, and diastolic blood pressure in 45 ASA PS I-II children between two and 16 years of age prior to elective tonsillectomy. The study groups were not significantly different with respect to age, weight, or gender. We were unable to detect a change in heart rate, systolic or diastolic blood pressure, or haemoglobin saturation following the rapid administration of ondansetron or metoclopramide. We conclude intravenous ondansetron or metoclopramide (for the prevention of postoperative vomiting) are not associated with cardiovascular instability when administered rapidly to healthy children prior to elective surgery.