Metabolic, haemodynamic and neuroendocrine responses to suxamethonium (SCh) were measured in five normal swine and five swine susceptible to malignant hyperthermia (MH), to compare the responses with those previously reported for halothane. Following SCh, the onset of MH was sooner and more abrupt than following halothane. The maximal changes in aerobic metabolism and body temperature sere similar, while the changes in lactate, potassium, hydrogen ion and catecholamine concentrations were smaller than those observed following halothane. These results are discussed in terms of the action of chemical depolarizing drugs such as suxamethonium and acetylcholine. The propagated muscle action potentials produce an increase in the free intracellular calcium concentration which may be self-regenerative, but which may become uncontrollable because of the peculiarities of MH that effect the calcium pump or storage areas.