DMSO inhibits the Na+, K+-ATPase, but stimulates the associated K+-phosphatase activity. For the ATPase, DMSO acts as an uncompetitive inhibitor toward both ATP and Na+, whereas it increases the K0.5 for K+. From measurements of the dissociation constant (Km) of these ions in the ligand states that correspond to the ATPase reaction, it can be shown that DMSO has little effect on the affinity for Na+, but decreases the affinity for K+ of the enzyme-phosphate intermediate (the form that has the highest affinity for K+). By contrast, DMSO decreases the Km for the phosphatase substrate (nitrophenyl phosphate) without affecting the Vmax. Moreover, DMSO decreases the K0.5 for K+ and also the Kd for K+ in the ligand states that correspond to the phosphatase reaction (which have only a moderate affinity for K+, since the acyl phosphate intermediate is absent in this pathway). These data may be incorporated into a reaction mechanism for the Na+, K+-ATPase. Initially the enzyme is phosphorylated to form an acyl phosphate intermediate, in steps that require Na+ and Mg-2+. At this stage the affinity of K+ is markedly increased (from the moderate affinity seen in the "free" enzyme and the phosphatase reaction). When K+ is bound, the phosphate group is transferred to the hydrolytic site where P-i is ultimately released. DMSO acts at the point at which the acyl phosphate group or the phosphatase substrate enters the hydrolytic site, inhibiting one and facilitating the other. At this stage the affinity for K+ is also changing, and DMSO apparently selects an enyme conformation of intermediate affinity. Ion transport may occur by a gate mechanism in an overall system that operates on a half-of-the-sites active enzyme pattern in which ATP hydrolysis may alternate between the dimeric subunits of the enzyme.