BACKGROUND: Standard base excess (SBE) is an important parameter for guiding fluid management in postoperative metabolic acidosis. However, individual SBE components, notably the chloride effect (Cl(eff)), provide valuable additional information. Cl(eff) is the deviation of the strong ion difference (SID) from normal caused by chloride loss or increase and represents the effect on SBE of an abnormal chloride-sodium ratio. Many centers use normal saline (NS) for intravascular volume therapy. In this study, we examined the impact of NS infusion on SBE and its chloride-driven component (Cl(eff)) in postoperative children. METHODS: The study was conducted in 119 children who underwent post-heart surgery in a Swiss pediatric intensive care unit. The 72-h postoperative course was divided into six observation periods, during which NS input and its impact on SBE and Cl(eff) were measured per period in each patient, and the results compared between patients infused and not infused with NS during each period. RESULTS: Normal saline was infused in 168/625 observation periods if indicated by volume deficit. Postoperative metabolic acidosis and the acidifying Cl(eff) were aggravated in the first 12 postoperative hours. Over the 72 h, NS infusion simultaneously lowered SBE by -0.06 mm x ml(-1) x kg(-1) body weight infused and Cl(eff) by -0.07 mm. CONCLUSIONS: Implementing serial Cl(eff) assessment could improve postoperative management by disclosing or excluding hyperchloremia as a cause of acidosis undetectable from SBE alone. Calculating the chloride-driven acidifying side effect of NS infusion using Cl(eff) improves the interpretation of SBE values and can optimize fluid management in postoperative metabolic acidosis.