A controversy exists in the literature as to the most appropriate sodium concentration for maintenance parenteral fluids. The purpose of this study was to evaluate urinary sodium composition in otherwise healthy children in order to help determine if 0.9% sodium chloride (NaCl) would be an appropriate parenteral fluid. The composition of urinary sodium was evaluated over 24 h in 100 otherwise healthy children aged 3-18 years referred to a pediatric nephrology outpatient clinic for hematuria or proteinuria. The average urine sodium concentration was 158+/-59 mEq/l, similar to that of 0.9% NaCl (154 mEq/l). Urine sodium excretion was 2.9+/-1.3 mEq/kg per 24 hours, and urine flow rate was 0.9+/-0.4 ml/kg per hour. It was concluded that healthy children generate free water via the excretion ofa hypertonic urine. It is unlikely that 0.9% NaCl would result in hypernatremia when administered in parenteral fluids.