The mode of action of dantrolene sodium was studied in 11 multiple sclerosis patients with spastic paresis of the legs by measurements of changes in electromyographic and mechanomyographic proprioceptive reflex responses and in voluntary power. Dantrolene sodium 0.5 mg per kg body weight given intravenously clearly reduced monosynaptic reflex twitch tension, but voluntary power only moderately so. The electromyographic reflex responses were unchanged or slightly increased. The mode of action of the drug on spindle function is discussed on the basis of the present findings and the literature. It is concluded that dantrolene sodium does not reduce spindle stretch sensitivity, but probably reduces activity in group II and tonic 1a afferent fibers.