In New Zealand white rabbits the right hind limb was immobilized in full extension with a plaster cast. The free left hind limb served as control. The masses of both the tetanic m. gastrocnemius and the tonic m. soleus considerably decreased as a consequence of immobilization for 5, 10, 14, 25 or 42 days. The decrease was more marked for the m. soleus. The water content of the muscle did not change substantially in the course of the atrophy. The total protein and myofibrillar protein contents of the immobilized muscles fell significantly. The tonic m. soleus atrophised sooner and was more extensive than the m. gastrocnemius. Superprecipitation of the myofibrillar proteins of the immobilized muscles decreased by 20-25% compared to the controls. The experimental model is considered suitable for further biochemical and ultrastructural investigations relating to the development of atrophy and to regeneration.