In the absence of Ca 2+, 20 mM hydroxylamine activates and in the presence of 20 μM Ca 2+ it inhibits the (Na + + K +)-ATPase (ATP phosphohydrolase, EC 3.6.1.3). Mn 2+, Cd 2+, Zn 2+, Pb 2+, and Cu 2+ can substitute for the Ca 2+-induced hydroxylamine inhibition at concentrations below 10 μM. Ba 2+, Sn 2+, Sr 2+, and Fe 2+ have no Ca 2+-like effect. This result is obtained in the presence of ethylene-bis-(oxyethylenenitrolo)-tetraacetic acid, a chelator with a high stability constant fr Ca 2+. If 8-hydroxyquinoline, a chelator with a low stability constant for Ca 2+, but a high stability constant for Cu 2+, is used instead of ethylene-bis-(oxyethylenenitrilo)-tetraacetic acid, Ca 2+ has no effect on the hydroxylamine-(Na + + K +)-ATPase. However, Cu 2+ at concentrations of 1 μM will induce inhibition of the hydroxylamine-(Na + + K +)-ATPase in the presence of 8-hydroxyquinoline. Hydroxylamine in the presence of Ca 2+ does not split the acylphosho intermediate of the (Na + + K +)-ATPase, but rather prevents the Na +-dependent phosphorylation of the enzyme. The mercaptans, 2,3-dimercaptopropanol and 2-mercaptoethanol have a biphasic effect on (Na + + K +)-ATPase. They inhibit at low concentrations (10–100 μM) and activate at high concentrations (above 1 mM). The inhibition is increased in the presence of 30 μM Ca 2+. Both mercaptans reverse the Ca 2+-induced hydroxylamine inhibition at concentrations above 1 mM. N-ethylmaleimide reverses the Ca 2+-induced hydroxylamine inhibition between 10–100 μM, if it reaches the enzyme before hydroxylamine. The reactivation apparently is due to alkylation of a sulfhydryl group different from the sulfhydryl group by which N-ethylmaleimide inhibits the (Na + + K +)-ATPase. The results are interpreted by assuming a heavy metal intrinsic to the enzyme and bound to a storage site. Hydroxylamine may decrease the affinity of the storage site for this heavy metal so it can bind to an inhibitory site. The latter site has no function by itself, but when it forms a complex with the intrinsic heayv metal, it acts