To find out why most patients with the nephrotic syndrome maintain a normal blood volume despite a reduced plasma colloid osmotic pressure (COP), we measured the transcapillary (plasma-tissue fluid) COP difference in 12 patients with the nephrotic syndrome, as well as in 6 patients during complete (n = 3) and partial (n = 3) recovery. Subcutaneous nylon wicks were used to collect tissue fluid. The albumin content was also measured. The albumin content and COP were lowered in both plasma and tissue fluid in the nephrotic phase, and rose gradually during recovery. During these changes the transcapillary COP difference only rose slightly: from 6.2 +/- 1.7 mm Hg when the plasma COP was below 10 mm Hg (n = 11) to 8.7 +/- 1.5 mm Hg when the plasma COP exceeded 20 mm Hg (n = 12). These observations indicate that in hypoproteinemia preservation of the intravascular volume is strongly dependent on maintenance of the difference in oncotic pressure across the capillary wall.