Halothane-induced contractures in isolated muscle fibers from swine susceptible to malignant hyperthermia (MHS) were significantly less when fibers were incubated in KRB plus 6.2 × 10−6M dantrolene sodium prior to the administration of 4 per cent halothane. Administration of dantrolene sodium at the time of maximum contraction to MHS fibers in which contractures had been induced by halothane significantly increased the rate of relaxation of these fibers compared with similar fibers not treated with dantrolene sodium. This study indicates possible prophylactic and therapeutic value of dantrolene sodium in malignant hyperthermia and suggests that the previously reported effectiveness of dantrolene sodium in preventing and treating halothane-induced contractures may be due, at least in part, to its direct effect on muscles.