A synthetic L-α-dipalmitoyl lecithin monolayer at an air-water interface was used as a model to study the effects of volatile anesthetics on cell membranes. Methoxyflurane, chloroform, halothane, enflurane and fluroxene were used in this study. When the surface pressure of the monolayer was kept constant, anesthetics at clinically effective tensions expaned the area 0.5 per cent When the area of the monolayer was kept constant, the surface pressure was increased about 1.0 dyn/cm by anesthetics. This increase of the surface energy was caused by the addition of 2.70 x 1013 anesthetic molecules to one square cm of the interface. The surface pressure-surface area curve shows a discontinuity, and a transition from liquid-expanded phase to liquid-condensed phase occurs at this point The liquid-expanded phase is regarded as the “melted” state of the monolayer. Anesthetics shifted the phase transition point towards a more condensed region, indicating melting of the monolayer membrane. Anesthetics decreased the latent heat and entropy change of the phase transition, implying that anesthetics facilitate the melting of the membrane. The compressional modulus, a measure of the rigidity of the monolayer, was decreased by anesthetics. This decrease of rigidity, or increase of fluidity, was also disclosed by analysis of the hysteresis curve (surface pressure-surface area) obtained by compression and expansion of the monolayer. The results support the unfolding theory of anesthesia which postulates that disordering and