Abstract The effects of various steroids and some aliphatic compounds on biological membranes (synaptosomal plasma membranes) were studied. It was shown 1. (1) Very dilute aqueous ‘solutions’ ( 1 · 10 −7 M to 1 · 10 −5 M ) are satisfactory for a study of such phenomena. 2. (2) When using amphipathic substances (e.g., octanol glucoside), the effect on membranes will depend on the relative concentrations of the reactants. At large concentrations of the amphipaths they act as detergents, solubilizing the membrane constituents through formation of mixed micelles, whereas at small concentrations such as those mentioned above, they would bind onto the membranes. 3. (3) The presence of a glycon in glycoside (e.g., cholesterol glucoside, dodecanol glucoside, etc.) is a convenient method of increasing their solubility in water. The sugar moiety does not contribute in their binding at specific sites. 4. (4) The specific activity of the functional parameter used in this study, i.e., the ouabain-sensitive ATPase, rises after 3 h preincubation with cholesterol or testosterone at least 2.5-fold above its original value. At this level, testosterone binds on synaptosomal plasma membranes in approximately 30-times smaller quantities per mg protein as does cholesterol. In the presence of both testosterone and cholesterol the effect of the two compounds is much higher. The glucocorticoid cortisol evokes a milder effect upon the specific activity of ouabain sensitive ATPase. 5. (5) Estrone and progesterone evoke decrease of the specific activity of the ATPase of synaptosomal plasma membranes from young dog brain (approx. 50%). This effect of the two hormones is thus opposite to that of cholesterol. 6. (6) At the membrane level, the steroid structure is not absolutely necessary to evoke enzymatic functional changes. Aliphatic compounds like dodecanol or its glucoside imitate the action of cholesterol or testosterone, while octanol or its glucoside simulate the action of progesterone. 7. (7) Accordingly, cholesterol and testosterone can be mutually excluded from their binding sites at proper relative concentrations, while octanol or its glucoside may exclude progesterone.