Albumin, transferrin, and total protein concentrations were measured in the mesenteric tissue, peritoneal fluid, and plasma of 12 ketamine-Nembutal-anesthetized Sprague-Dawley rats. Tissue samples were obtained with an 8-mm trephine; tissue water content was determined by a microgravimetric method to be 5.2 +/- 0.3 microgram water/microgram dry wt. Peritoneal fluid was collected by capillary action in hematocrit tubes, and blood samples were taken from a femoral artery catheter. Total protein concentrations of plasma (5.8 +/- 0.3 g/dl) and peritoneal fluid (2.6 +/- 0.1 g/dl) were determined by Lowry assay. Ratios of peritoneal fluid and tissue densitogram areas to plasma area were used to calculate total protein content of peritoneal fluid (2.5 +/- 0.1 g/dl) and tissue (1.8 +/- 0.2 g/dl). Albumin concentrations were 1.1 +/- 0.1 g/dl for tissue, 1.4 +/- 0.1 g/dl for peritoneal fluid, and 2.8 +/- 0.1 g/dl for plasma. Transferrin concentrations were 0.09 +/- 0.01 g/dl for tissue, 0.13 +/- 0.01 g/dl for peritoneal fluid, and 0.28 +/- 0.01 g/dl for plasma. Peritoneal fluid protein concentrations were similar to values found for lymph in previous studies. Protein concentration in the tissue buttons was significantly less than that of peritoneal fluid. This contradicts the widely held assumption that the protein concentration of fluid outside the matrix is representative of a well-mixed interstitial matrix fluid protein concentration.