1. The action of the skeletal muscle relaxant drug, dantrolene sodium, given intravenously, on the intrafusal fibres of the soleus muscle of the urethane-anaesthetized rat has been investigated. The experiments were made on functionally single spindle afferents and gamma fusimotor fibers isolated in dorsal and ventral roots respectively. 2. Dantrolene sodium was without effect on the discharge of primary and secondary afferents from the passive muscle spindle, nor were the dynamic indices of these endings affected. 3. Intrafusal muscle contraction was measured indirectly by means of the spindle afferent discharge. 4. The intrafusal muscle twitch contraction, as measured by means of the amplitude of the frequencygram, was depressed more slowly to a lesser extent than was the twitch of the extrafusal contraction. 5. Intrafusal contraction resulting from tetanic stimulation of the gamma fibre was depressed by dantrolene sodium to an extent dependent upon the stimulation frequency. At frequencies of 10, 25 and perhaps 50 Hz the depression was complete, that is, no afferent response was evoked; at 200 Hz stimulation, the depression was minimal (or non-existent). 6. For a muscle spindle primary ending under dynamic gamma activation dantrolene sodium caused a reduction of dynamic index whereas for the ending under static activation it caused an increase. 7. The significance of the findings in terms of the clinical use of the drug is considered.