Aliphatic alcohos produce changes in the structural arrangement of lipids in bilayers as indicated by studies using the steroid spin probe 3- spiro-2′-( N-oxyl-4′,4′-dimethyloxazolidine)) cholestane. The concentrations of alcohol corresponding to the onset of perceptible changes in organization correlate well with those causing anesthesia in biological systems, suggesting that anesthesia may be caused by small disruptions of lipid bilayer structure in biological membranes. The data indicate also that lipid bilayer disruption or reorganization is involved in alcohol effects on black lipid membrane conductivity and erythrocyte antihemolysis.