1. General anaesthetics induce reversible structural and functional modifications of the proton pump bacteriorhodopsin contained in the purple membrane of halobacteria. 2. This short paper describes the phenomenological similarity between the effects of anaesthetics and heating on this system. 3. pH dependence and thermal analysis suggest that the structural change taking place at room temperature in the presence of anaesthetic is the same as that observed at temperatures higher than 70 degrees C in the native membrane. In addition, part of the structural modification has an irreversible character when induced by anaesthetic as well as by heating.