Rats (80-100 g) were fasted for 48 hours prior to receiving i.v. infusions of [(14)C]tyrosine (0.5 ml/hour)in 0.9% saline or 2.8 M glucose or 2.5 M glucose with 0.1 M each of leucine, isoleucine and valine. The incorporation of [(14)C]tyrosine into proteins of different muscles was measured at the end of six hour infusions. Heart, soleus and diaphragm muscles incorporated more tyrosine into proteins than psoas and gastrocnemius under all conditions. Infusions of glucose alone did not accelerate tyrosine incorporation significantly in the muscles tested. The addition of branched chain amino acids to glucose markedly increased tyrosine incorporation into proteins in the diaphragm, and to a lesser extent in soleus and psoas. No effect was detectable in heart and gastrocnemius. Previous reports of differences in protein synthetic rates between different types of muscles are confirmed. Additionally, the data suggest that branched chain amino acids may be rate limiting for muscle protein synthesis in fasted rats in vivo, and that the responsiveness to branched chain amino acids differs between muscles.