The incorporation of14 C-glycine into the plasma proteins of rats at 3, 10, and 30 days following whole-body irradiation with fast neutrons has been measured. Only at 3 days postirradiation did the specific activity of the proteins from the irradiated rats exceed that of their pair-fed controls. The early decrease in concentration of plasma proteins was shown to result from anorexia, while the late (30 days) decrease seemed more likely result from damage to the synthetic process. At all time intervals, the release of labeled proteins following the injection of14 C-glycine was more delayed in the irradiated rats.