Background: Fluid therapy is essential for safe perioperative management. Numerous reports of serious complications as a result of inappropriate fluid management, including brain damage and death of children, have been published. The aim of this study was to assess the effects of intraoperative fluids on serum glucose and electrolyte concentrations as well as serum osmolality. Methods: 91children, ASA I and II, undergoing elective ENT surgery were enrolled to this prospective, randomized, open-label study. They were randomly assigned to receive: group G5W: 5% glucose in water solution, group GNaCl: 3.33% glucose in 0.3% NaCl, and group RA: Ringer's acetate. Serum glucose, sodium, potassium, phosphate concentrations and serum osmolality were analysed before the induction of anaesthesia, immediately following the completion of surgery and 60 minutes after that. Results: Postoperative hyperglycaemia was observed in 94% of children in group G5W and in 37% of group GNaCl. In all the groups glucose concentration increased significantly after surgery. Postoperative hyponatraemia occurred in 36% of patients in the group G5W, and in 3.7% in the group GNaCl. Neither hyperglycaemia nor hyponatraemia occurred in group RA. Postoperative osmolality decreased significantly in groups G5W and GNaCl and remained unchanged in group RA. Conclusions: As Ringer's acetate did not cause significant changes in glucose and electrolyte concentrations, it seems to be the safest for intraoperative use in children undergoing elective surgery. As hypotonic fluids may cause hyperglycaemia and hyponatraemia, they should be avoided intraoperatively.