Abstract Regional differences in plasma extravasation during endotoxin shock in rats and a possible relationship with changes in regional blood flow were studied with radioactive isotopes ( 125I-HSA, 51Cr-labeled red blood cells, microspheres) in anesthetized rats (pentobarbital). Shock was induced by intravenous infusion of endotoxin ( Eschericia coli; 10 mg·kg −1) for 60 min (starting at t = 0); at t = 120 min, the experiments were terminated. These rats ( n = 8) were compared with time-matched control rats ( n = 8). A third group (rats killed 7.5 min after injection of 125I-HSA, i.e., no extravasation; n = 8) served as baseline. The amount of plasma extravasated in 2 hr of endotoxin shock was significantly increased over control values in skin (by 67%), colon (88%), skeletal muscle (105%), stomach (230%), pancreas (300%), and diaphragm (1300%). Losses of 125I-HSA into intestinal lumen and peritoneal cavity had also increased over control values by 146 and 380%, respectively. Blood flow was compromised in most organs except heart and diaphragm. Extravasation when normalized for total plasma supply was correlated with total blood supply: the more the blood supply decreased, the higher the normalized extravasation. In the diaphragm, however, blood supply and plasma leakage increased together. Decreased blood supply and plasma extravasation may be related but they could also be simultaneously occurring independent phenomena with a common origin.