The effects of activated platelet (Pit) on muscle degradation were investigated, employing the in vivo disseminated intravascular coagulation (DIC) model induced by thrombin injection and the in vitro tissue culture system of skeletal muscles in rats. Both the release of tyrosine and leucine into the culture medium during 2 h incubation from the muscles harvested 30 min after thrombin injection increased by about 50% compared with control muscles. The addition of thrombin-activated platelet supernatant (TAPS) significantly increased the release of leucine into the incubation medium of the soleus muscles dissected from normally fed rats by 31 % in comparison with the respective controls. No significant effect was observed in terms of the release of tyrosine or leucine from the incubated muscles by aspirin treatment before obtaining TAPS, or by the addition of thrombin itself up to the concentration of 0.67 µm/ml which was contained in the incubation medium of TAPS. These data suggest that protein catabolism is accelerated in the muscle from the thrombin-treated rats exhibiting DIC. The supernatant of activated Pit might contain a factor which modulates protein metabolism. That factor is different from prostaglandins or thrombin. Thus, active consumption of Pit may contribute to an increase of muscle breakdown in various catabolic states