The mechanism of increase in the protein concentration of ascitic fluid during diuresis was investigated in 27 patients with chronic liver disease. The albumin concentration increased in ascites from .58 +/- .49 gm.% in the initial paracentesis to 1.48 +/- .69 gm.% in the final paracentesis (P less than .001) as the serum albumin concentration increased from 2.40 +/- .44 gm.% 2.94 +/- .56 gm.% (P less than .001). The serum to ascites albumin concentration gradient decreased significantly from 1.82 +/- .39 gm.%-1.46 +/- .45 gm.% (P less than .001). Despite this decrease, the serum to ascites albumin concentration gradient was relatively constant (decrease of .36 gm.% or 20% of initial value) compared to either the absolute or percentage change in ascites total protein concentration (increase of 1.48 gm.% or 107% of initial value). In four patients studied prospectively, the plasma volume did not change (3.53 +/- .80 1.-3.73 +/- .59 1.) during diuresis, despite increase in total intravascular albumin (85.7 +/- 25.6 gm.-99.9 +/- 22.3 gm.; P less than .05) and decrease in total albumin in combined intravascular and peritoneal compartments (156.0 +/- 30.1 gm.-143.4 +/- 35.3 gm.; P less than .05). Since neither concentration without decrease in plasma volume or synthesis of protein without increase in total compartmental protein appear to be the major mechanism of serum protein concentration increase, the increase in serum protein concentration was attributed to redistribution of protein from ascites to the intravascular compartment. The increase in ascitic fluid protein concentration during diuresis can be attributed to an increase in serum protein concentration in the presence of the relatively stable serum to ascites albumin concentration gradient.