Arginine vasopressin (AVP), atrial natriuretic factor, and water balance were examined in the infants with or without hronchopulmonary dysplasia (BPD) during the first 4 wk of life. Fourteen premature infants, nine in the early stage of BPD secondary to respiratory distress syndrome (BPD infants) and five healthy low birthwt infants (LBW infants), were the subjects of this study. The water and sodium balance, renal function, and plasma AVP and atrial natriuretic factor concentrations were determined during the first 4 wk of life. Plasma AVP and atrial natriuretic factor levels of BPD infants at the 4th wk of life were higher than those of LBW infants at the corresponding age. Urine osmolality was higher and free water clearance was lower in BPD infants at the 4th wk of life when compared with each parameter in LBW infants, respectively. PaCo2of BPD infants at the 4th wk of life was more elevated than that of LBW infants. These results suggest that elevated plasma AVP level may be related with pulmonary abnormalities and that atrial natriuretic factor may hence compensate the water retention resulted from the functionally activated AVP in the early stage of BPD.