The water flea Daphnia magna was studied as a potential model of the potency of inhalation anaesthetics in humans. Anaesthesia was defined as lack of movement to a noxious stimulus (strong light). Effective concentrations which produced anaesthesia of 50% of the individuals (EC50) for halothane, isoflurane and enflurane were 1.006, 1.156 and 1.415, respectively. Anaesthesia was reversible rapidly. It is suggested that Daphnia should replace vertebrates in early studies of inhalation anaesthetic potency.