Rat fibrinogen, labeled with 1311, was used to determine the catabolic rate of fibrinogen in rats fed a thrombogenic butter-cholesterol diet. Despite a significant increase in plasma fibrinogen levels from 200 to 300 mg/100 ml in the experimental group, the percent of the total fibrinogen pool broken down per day did not change significantly. The calculated replacement of fibrinogen in mg per day rose about 30%. Since the fibrinogen pools of these animals appeared to be in a steady state over the 4 days of observation, the results indicate that synthesis as well as catabolism of fibrinogen was somewhat increased by the thrombogenic diet. Possible explanations for the apparent variation of these results from those of others suggesting markedly depressed synthesis in similar animals are discussed.