Summary 1. Intravenous injection of Cr51 tagged endotoxin into mice resulted in marked increase of circulating endotoxin as dosages reached the lethal range. 2. Distribution pattern of endotoxin in mice injected intracranially was similar to that of mice injected intravenously. 3. Distribution pattern of endotoxin in mice injected intramuscularly differed markedly from those found in mice injected intravenously or intracranially. 4. Premedication of mice with trypan blue resulted in greater susceptibility to endotoxin and increased amounts of circulating endotoxin in mice injected intramuscularly or intravenously. 5. Using the technique described, lethality of endotoxin for mice could not be related to any one distribution pattern.