The control by nucleotides of the Ca2+ -activated channel which regulates the nonspecific permeability of the mitochondrial inner membrane has been investigated quantitatively. The cooperative binding of two molecules of ADP to the internal (matrix) side of the channel causes a mixed-type inhibition of channel activity. ATP, AMP, cAMP and GDP are all ineffective. NADH shows a pattern of inhibition similar to that of ADP, though the apparent KI is higher by a factor of 200. NADPH relieves the inhibition by NADH. NAD+ also inhibits, b,t its affinity is a factor of 10 less than that of NADH. When ADP and NADH are added together, they act synergistically to inhibit the Ca2+-activated channel. It is concluded that the concept of the modification of enzyme activity by the allosteric binding of nucleotides, which is well established for soluble enzyme systems, also has application to the regulation of channels that control membrane permeability.