It is not known whether proteins or lipids are the primary target of anesthetic action. The resolution of this problem is hampered by the fact that it is not possible to investigate the biological activity of integral membrane proteins in the absence of lipids. However, certain characteristics of membrane protein function inhibition by anesthetics cannot be explained on the basis of an indirect inhibition by disturbance of the lipid bilayer and, therefore, most likely are the result of a direct anesthetic-protein interaction. This is the case (i) when the anesthetics competitively interfere with the binding of an endogenous ligand to the membrane protein and (ii) when the size of the anesthetic molecule is of importance for the potency and/or mechanism of inhibition. The present study shows that this is true for a membrane transport system, the Na+/K+/Cl- cotransport in glial-type cells.