Changes in fluid, electrolyte and acid-base balance were evaluated in 8 patients during extracorporeal circulation. H2O increase and osmolarity and colloid osmotic pressure decrease, due to priming and perfusion technique used for cardiopulmonary by pass, were found. Mild respiratory alkalosis was also present. K+ serum, because of KCL supplements, was always maintained within the normal values, while Na+ serum concentration was low, despite of large amounts of Na+ infused during perfusion. After examination of disturbances in fluid, electrolyte and acid-base balance, the Authors report the performed corrections and emphasize the importance of a prompt recognition and the immediate therapy of these changes to avert serious cardiac impairments during extracorporeal circulation or during the immediate postoperative period.