1. The effects of dimethothiazine have been studied on the response of afferent fibres from primary and secondary endings of muscle spindles in the soleus muscle of the decerebrate cat during stretching of the muscle under controlled conditions.2. Dimethothiazine in doses of 1 to 4 mg/kg intravenously reduced the discharge frequency of primary and secondary endings. Higher doses of dimethothiazine had little further significant effect on the discharge frequency.3. The discharge frequency recorded from de-efferented muscle spindles in soleus of the decerebrate cat were similar to the discharge frequencies obtained in preparations with intact ventral roots which had received a high dose of dimethothiazine.4. Dimethothiazine had little significant effect on the discharge frequency of afferent fibres from muscle spindles in the soleus muscle of decerebrate preparations where the ventral roots were cut.5. These effects of dimethothiazine on muscle spindle activity were not related to any changes in blood pressure of the decerebrate cat.6. Dimethothiazine appears to reduce the effects of both the dynamic and static fusimotor fibres on the spindle.7. The doses of dimethothiazine which effect spindle discharge frequency are similar to those required to reduce decerebrate rigidity.