Slow soleus muscle in guinea pig developed within 4--6 days after local injection of a sublethal dose of tetanus toxin and 2--4 days after the first signs of local tetanus, a myostatic contracture characterized by a change in the passive tension--lengthening curve associated with a considerable decrease of the sarcomere number. It was demonstrated by clinical and EMG investigations that the soleus did have a tetanic spasm at least within the 2--4 day period of observation. When local tetanus was confined to slow soleus by functional suppression of rapid gastrocnemius and ankle flexor muscles, the decrease of the sarcomere number still persisted. This decrease failed to occur after section of the nerve supplying the soleus when associated with an injection of the tetanus toxin, and was much greater than when the soleus was passively shortened for the same period of time by plaster cast.