Breathing behaviour is driven by chemosensory information sensed by oxygen or carbon dioxide receptors that detect the level of these substances either internally or in the external environment. In terrestrial species, oxygen chemosensation is primarily through internal sensors, whereas in aquatic animals, such as the pond snail Lymnaea stagnalis, external oxygen chemo - receptors are thought to be more important, due to the low partial pressure of oxygen in water. It has been hypothesized that an external chemosensory organ, the osphradium, modulates aerial respiratory behaviour in hypoxic conditions in Lymnaea, but recent data indicate that this may not be the case. In the present study, we removed the input from this organ to the central nervous system and measured aerial respiratory behaviour. We assessed breathing behaviour prior to surgery and then operated on the snails, severing either (1) the osphradial nerve proximal to the osphradium or (2) the right internal parietal (RIP) nerve into which the osphradial nerve, in addition to other axons from the lung/pneumostome area, projects. We also used a sham-operated control group. Severing either the osphradial or RIP nerve did not alter breathing behaviour in eumoxic or hypoxic conditions relative to the behaviour prior to surgery or that of sham-operated animals. Therefore, we conclude that input from the osphradium does not drive aerial respiratory behaviour.