The authors discuss the problem of administration and amount of fluids and electrolytes in neonates after operations of inborn developmental defects and during the postoperative period. Parenteral nutrition is considered by the authors an important component of comprehensive intensive care which influences in a marked way the results of the surgical operation. As regards the control of water homeostasis they analyze factors which control the hydration. Next the authors discuss the daily intake of ions; some space is devoted to sodium the main cation of extracellular fluid. Next the authors discuss this aspect of parenteral nutrition as practiced at the Clinic for Paediatric Surgery in Brno.