Studies of the effects of inhalation anesthetics on membrane carrier systems were carried out using, as a prototype, entry of monosaccharides into human erythrocytes. The data obtained lead to the proposal that, by acting as penetrating competitive inhibitors in a system which involves uphill counter-transport mechanisms, anesthetics can result in either acceleration or inhibition of the rate of transport of monosaccharides across cell membranes, depending upon the relative concentration of the anesthetic.