Fractional peritoneal reabsorption rates (FPRR) were determined from the plasma activity after simultaneous intraperitoneal injection of 131I-labelled serum albumin (a) and 125I-labelled immunoglobulin G-IgG (g) in eight patients with cirrhosis (+ ascites 6, -ascites 2) and in one patient with carcinomatous ascites. Trans-vascular escape rates of albumin (TERa) and IgG (TERg) were determined in the cirrhotic patients from the disappearance of simultaneously intravenously injected 131I-labelled serum albumin and 124I-labelled IgG. Peritoneal space to plasma appearance times ranged 0.1-3.3 h, and the appearance times of albumin and IgG were almost identical. In patients with cirrhosis FPRRa and FPRRg were on average 1.27 and 1.21% of intraperitoneal protein masses returning to plasma per hour, respectively. Mean FPRRg/FPRRa ratio was 0.95 and this value was not significantly different from unity, but significantly higher (P < 0.001) than the ratio between the free diffusion coefficients of IgG and albumin (0.06). The calculated ascitic fluid flow rate was on average 61 ml/h. TERa and TERg were on average 9.6 and 8.6% of intravascular protein masses per hour, mean TERg/TERa ratio was 0.95. Peritoneal space to plasma protein flux averaged 0.4% of the intravascular protein mass per hour. The results point to filtration (convective flux) as the main transport mechanism responsible for protein passage into the peritoneal cavity as well as for the protein passage (lymphatic drainage) back into the plasma. Pressure measurements during catheterization confirmed pressure differences essential for convective flux.