Recent advances in the study of cell membrane structure and function confirm the Singer-Nicolson model of a lipid bilayer in which membrane proteins are embedded. Mobility of these proteins is essential for biological activity. The Ca2+-ATPase of skeletal muscle is a typical example of a transmembrane integral protein. Within this protein various domains can be identified on the basis of their functional activities. A hydrophobic pocket within the APTase polypeptide appears to be a possible site of interaction with halothane molecules. These findings may explain the well-known correlation between anaesthetic potency and lipid solubility with specific inhibition of functions known to be mediated by membrane proteins.