The actions of three volatile general anaesthetics on withdrawal responses of the pond-snail Lymnaea stagnalis (L.).

1. The gastropod mollusc Lymnaea stagnalis (L.) is an ideal model system for studies on anaesthesia. It is reversibly anaesthetized by the general anaesthetics halothane, enflurane and isoflurane. 2. Criteria for "anaesthesia" in Lymnaea were established. The reflex used in ED50 trials was the whole animal withdrawal reflex. 3. ED50 values for halothane, enflurane and isoflurane were, 0.83% v.v. (volume for volume), 1.01% v.v. and 1.09% v.v. respectively. 4. Relationships between anaesthetic concentrations, weights of animals and mortality rates are reported.