Anaesthesia is a drug-induced reversible perturbation of neuronal activity. Since a wide variety of structurally unrelated substances are capable of producing this phenomenon, it has been generally accepted that anaesthetics produce their effects through non-specific hydrophobic interactions. Results of recent studies in whole animal and cellular (membrane) preparations demonstrate that a unitary theory of action does not exist. Anaesthetics can produce a spectrum of activity in the central nervous system, and different agents produce different patterns of activity. At the cellular and membrane level, differential effects have been observed, structural dependent differences occur and optical isomers display very different activities. The perturbation (fluidity change) of membrane components does not appear to be uniform for all anaesthetics. It is concluded that anaesthetics are selective agents, and produce their effects at multiple sites and through a variety of mechanisms.