Background: Hyponatremia is the most common electrolyte imbalance in clinical medicine. About 15% of hospitalized patients have hyponatremia. The type of administered intravenous fluid may lead to changes in serum sodium concentrations. Objectives: The aim of this study was to compare the effect of 1/3 2/3 fluid versus dextrose 5% half saline as intravenous maintenance fluid on serum sodium levels. Methods: In this cross sectional study, children aged 2 months to 12 years old admitted to the general pediatric ward of Bushehr Persian Gulf Shohada hospital were evaluated. They were randomly assigned to take 1/3 2/3 fluid (with 51 mEq/Lit sodium) or dextrose 5% half saline (with 77 mEq/Lit sodium). Serum sodium levels were checked before and 24 hours after the initiation of the IV fluid therapy. Results: A total of 295 patients were enrolled in this study and randomly received 1/3 2/3 fluid (N=152) or dextrose half saline (N=143). In children aged less than 5 years, there was a significant change in serum sodium levels after administration of 1/3 2/3 fluid (P=0.011 in <1 year old and P<0.001 in 1-5 year old). However, serum sodium was still in the normal range. This change was not significant in children above 5 years old. In children taking dextrose half saline, serum sodium levels did not change significantly (P=0.144). Conclusion: In a group of pediatric patients admitted to general pediatric ward, both 1/3 2/3 solution and dextrose 5% half saline are safe.