The effect of azide on the activity and phosphorylation of ATPase in rat liver mitochondria was studied. It was shown that in the absence of exogenous Mg2+ azide inhibits both the activity and phosphorylation of ATPase. In the presence of exogenous Mg2+ azide inhibits ATPase hydrolysis, but does not inhibit the enzyme phosphorylation. It is concluded that the one-sided effect of azide on the activity of the ATPase (ATP-synthetase) mitochondrial complex is realized by participation of Mg2+. It was found also that the inhibitory effect of azide depends on the exogenous Mg2+ concentration.