Rates of synthesis of plasma albumin and fibrinogen were measured by the [(14)C]carbonate method in normal rabbits and in animals that received a single intravenous injection of Shigella endotoxin 14-48hr. earlier. 2. The accuracy of the method was improved by introducing refinements into procedures for measuring (14)C radioactivities associated with both urea and proteins that are lost from the plasma during the synthesis interval. 3. The synthesis interval (time between injecting carbonate and measuring specific radioactivities of protein guanidine carbon in plasma) can be shortened with advantage to 3-4hr. 4. Injection of endotoxin markedly decreased the fractional rate of loss in the first few hours of injected radioiodine-labelled fibrinogen and to a smaller extent of similarly labelled albumin from the plasma. The absolute rate of synthesis of fibrinogen was increased in endotoxin-treated rabbits by more than 400% compared with normal animals, and the rate of synthesis of albumin was increased by about 60%.
