A new ATP analog, adenosine‐5‐N′‐(2,4‐dinitro‐5‐fluorophenyl) phosphohydrazide (DNPH‐AMP), has been synthesized, which is an irreversible inhibitor of Na,K‐ATPase. Interaction of the analog with the enzyme in the presence or K+ is describe by the scheme:, and corresponding kinetic constant k3 and Ki are found equal to 2.5 min−1 and 1.6 mM. In the presence of Na+ the time course enzyme inactivation by DNPH‐AMP is a biphasic curve in the semilogarithmic plot, The k3 and Ki values calculated for this case according to Fritzsch (Fritzch (1985) J. Theor. biol, 117 397) are equal to 2.45 min−1 and 2.5 mM, respectively. ATP transforms the K* ‐type or Na,K‐ATPase inactivation into the one that takes place in the presence of Na*.