Dorsiflexion of one unloaded hind limb caused hypertrophy of the soleus relative to weight-bearing controls and faster growth of the plantaris and gastrocnemius muscles relative to the contralateral freely moving muscles. Unloading of the soleus muscle diminished primarily myofibrillar proteins whereas stretching increased all proteins. Stretching the soleus increased RNA, accelerated, especially, in vitro synthesis of sarcoplasmic proteins, and diminished in vitro proteolysis. Both in vivo and in vitro results showed slower synthesis and faster degradation in the freely moving than in the weight-bearing soleus muscle, faster synthesis and slower degradation in the stretched than in the freely moving soleus muscle, and faster degradation in the stretched than in the weight-bearing soleus muscle. Hence, stretching of the soleus muscle prevented changes in mass and protein metabolism produced by unloading. Shortening of the extensor digitorum longus muscle produced less muscle growth, slowed in vitro protein synthesis, and lowered RNA relative to the contralateral, freely moving muscle.