The effectiveness of intravenous administration of dantrolene in prevention and treatment of fulminant malignant hyperthermia (MH) initiated by halothane and succinylcholine (SCh) in genetically susceptible swine was assessed. In six animals, prior administration of dantrolene in doses of 5mg/kg or more prevented MH, while 1 or 3 mg/kg attenuated MH, and 0.1 mg/kg had no effect. In ten additional swine, therapy was not started until MH was fulminant. Five of these were then given supportive therapy only (discontinuation of anesthesia, hyperventilation with oxygen, surface cooling, and NaHCO2). The remainig five received the same supportive therapy, plus dantrolene (7.5 mg/kg). With supportive therapy only, arterial bloodpH, Pco2and Pco2retumed toward normal, but oxygen consumption (VO2), blood lactate, potassium (K*). catecholamines, and temperature continued to increase and the course of MH was unaltered. When dantrolene was added to supportive therapy, VO2, lactate, K*, catecholamines, and temperature decreased, and the course of MH was dramatically slowed and, apparently, reversed.