The present study is a prospective, randomized, double blind, controlled study with parallel groups. The planned investigation is designed to compare the efficacy of an ORS with L. reuteri DSM 17938 and zinc to an ORS with similar osmolarity and content of salts but without L. reuteri DSM 17938 and zinc, on the duration and severity of acute gastroenteritis. 92 children aged 6-36 months seen at private pediatric clinics and/or at the emergency clinics of the Athens Childrens Hospital "AGIA SOPHIA", and treated either as outpatients or as inpatients will be recruited until the final sample size is reached. Assuming a difference of 30% between groups in the primary outcome of prevalence of diarrhoea on day 2, and estimating an attrition rate of approximately 15%, the final sample size will be 92 subjects, or 46 subjects in each arm.