Exposed regions of the α- and β-subunits of membrane-bound Na+,K+-ATPase were in turn hydrolyzed with trypsin. Resistance of the β-subunit to proteolysis was shown to be due mainly to the presence of disulfide bridge(s) in the molecule. A model for the spatial organisation of the enzyme in the membrane was proposed on the basis of detailed structural analysis of extramembrane regions of both subunits. © 1987.