Low concentrations of general anesthetics, including halothane, ethrane, trilene, diethyl ether and chloroform are observed to shift the phase transitions of phospholipid vesicles to lower temperatures, and from these data partition coefficients for the anesthetic between lipid and water can be calculated. In contrast to the anesthetics, high concentrations of ethanol are required to shift the phase transition of lipids and glycerol causes no effect. Above the phase transition general anesthetics alter nuclear magnetic resonance spectra of phospholipid dispersions and increase the rotational and lateral diffusion rates of fluorescent probes located in the hydrocarbon core of the bilayer, indicating that they induce disorder in the structure. In red blood cell membranes and sarcoplasmic reticulum fragments, the rotational diffusion rate of 1-phenyl-6-phenylhexatriene is increased in the presence of general anesthetics. The 220 MHz nuclear magnetic resonance spectra of sarcoplasmic reticulum reveal some resolved lines from the lecithin fatty acid protons; addition of general anesthetic increases the contribution of these peaks. The data from the NMR and fluorescence techniques lead to the conclusion that general anesthetics increase the pool size of melted lipids in the bimolecular phospholipid layers of biological membranes; this would account for the ability of general anesthetics to increase passive diffusion rates of various substances in membranes.