This clinical trial is intended to evaluate the efficacy and safety of the administration of ondansetron in children with mild-moderate dehydration.

The target population will be children between 4 and 13 years old who present vomiting.

Inclusion Criteria:

• Presence of at least 2 vomits in the last 4 hours.
• Negative tolerance test.
• Signature of the informed consent by the parents or legal guardians.
• Diagnosis of vomiting or diagnosis of acute gastroenteritis with vomiting.
• Patients weighing 15 kg or more.

Exclusion Criteria:

• Altered conciousness.
• Sospected acute abdomen (appendicitis, invaginations...).
• Suspicion or presence of obstructive disease.
• Known severe digestive disease (Chron´s disease, ulcerative colitis...).
• Suspected food poisoning.
• Allergy to any of the drugs used in the study.
• Severe dehydration.
• Bilious vomiting.
• Previous abdominal surgery.
• Pathology that may affective degree of hydration of the patient (kidney failure, hypoalbuminemia..).
• Admission requirement or intravenous rehydration for any reason.
• Concomitant use of apomorphine.
• In relation to the possible lengthening of the QT, patients with problems of previous arrhythmia should be excluded or that they are receiving concomitant medication that can lengthen the QT, that have electrolyte disturbances.
• Patients receiving other serotonergic drugs.
• Patients who have undergone a previous adenotonsillectomy.
• Patients with hypocalcemia and/or hypomagnesemia.
• Patients with hereditary fructose intolerance.
• Diabetes patients.
• Lactose intolerant patients.
• Patients with indication of the low sodium diets.
• Hypersensitivity to ondansetron or other antagonists of the 5-HT3.