The acute effects of a single intravenous dose of L-asparaginase on protein synthesis were studied in normal rabbits and in animals that had received turpentine to stimulate fibrinogen production. Male New Zealand rabbits received L-asparaginase (500 U/kg) 16 hr before the injection of the radiolabeled amino acid [75Se]selenomethionine (75SeM). Incorporation of 75SeM into fibrinogen and serum proteins in the L-asparaginase-treated rabbits was the same as for saline-treated controls, with fibrinogen representing approximately 5% of the labeled plasma proteins. In turpentine-treated rabbits, the maximal incorporation of 75SeM into serum proteins remained unchanged, whereas 75SeM-fibrinogen increased sixfold and accounted for 25% of the labeled proteins. Animals that received L-asparaginase at the same time as turpentine or 14 hr later showed significant decreases in synthesis of both serum proteins and fibrinogen. 75SeM-fibrinogen that was purified from L-asparaginase-treated rabbits underwent normal catabolism when injected into normal recipient rabbits. These data indicate that L- asparaginase can acutely cause partial inhibition of both serum protein and fibrinogen synthesis when administered to rabbits shortly before or during a period of increased fibrinogen production. Fibrinogen that is synthesized in the presence of L-asparaginase does not have an abnormal rate of catabolism.