The effect of decreases in plasma colloid osmotic pressure (COP) on the development of lung edema was studied in nephrotic rats. Five control animals were compared with five animals with nephrotic syndrome induced by administration of anti-rat glomerular basement membrane antibody. The COP and the left ventricular diastolic pressure (LVDP) were significantly decreased in nephrotic rats. However, no significant differences were seen in the COP - LVDP gradient, weight-to-dry lung weight ratio, or arterial oxygenation between the nephrotic and control animals. These data suggest that decreases in COP during nephrosis are not associated with accumulation of extravascular lung water.