Serum creatine phosphokinase was estimated at induction and 24 hours postoperatively in 79 children undergoing ophthalmic operations. The children received the following five anaesthetic sequences: (1) halothane-suxamethonium-halothane; (2) thiopentone-suxa-methonium-halothane; (3) diethyl ether-suxamethonium-halothane; (4) halothane alone; (5) diethyl ether alone. Each group of patients receiving suxamethonium showed a significant rise in serum creatine phosphokinase while groups in which suxamethonium was omitted failed to show significant change. It is therefore concluded that suxamethonium is the most important agent for the release of creatine phosphokinase from muscle during anaesthesia. The anaesthetic induction agents used before suxamethonium influenced this release to a certain extent. Halothane in particular increased it. The possible relationship between changes in serum creatine phosphokinase and malignant hyperthermia is discussed.