Transient tachypnea of the newborn (TTN) results from delayed clearance of lung fluid, which is mainly dependent on lung epithelial Na+ channel (ENaC) function. However, although fluid and Na+ clearance is mainly regulated by ENaC-mediated renal tubular function, renal responses to this disturbed pulmonary fluid and Na+ balance are not clearly defined. Therefore, we aimed to investigate the renal fluid and Na+ balance by the amount of diuresis and Na+ excretion in infants with TTN. Newborn infants with TTN were included in the study group, while healthy newborn infants served as the control group. All infants received standard nursery care and fluid volume. Daily total urine volume and fractional excretion of Na+ (FENa) were calculated at the end of the first day of life. In infants with TTN, mean daily total urine volume (ml/kg/day), mean FENa and weight loss percentage were significantly lower when compared to the control group (24.2±2.6 ml/kg/day vs 27.2±5.1 ml/ kg/day, 0.5±0.2 vs 0.6±0.2 and 5±1.1% vs 8±1.3%, respectively, p<0.05). This result was consistent with the result of a recent report about decreased serum levels of atrial natriuretic peptide (ANP) in infants with TTN. This decreased level of ANP may lead to inadequate inhibition of ENaC in renal tubular cells and decreased diuresis and natriuresis. In conclusion, decreased diuresis and natriuresis may have a pathophysiological importance in TTN.