TRPM7 channels are nonselective cation channels that possess a functional alpha-kinase domain. It has been proposed that heterologously expressed TRPM7 channels are activated (Runnels et al., 2001) or inhibited (Nadler et al., 2001) by dialyzing the cell with millimolar levels of ATP. The endogenous correlate of TRPM7 has been identified in T-lymphocytes and RBL (rat basophilic leukemia) cells and named MagNuM (for Mg(2+)-nucleotide-inhibited metal) or MIC (for Mg(2+)-inhibited cation). Here, we report that internal Mg(2+) rather than MgATP inhibits this current. Cytoplasmic MgATP, supplied by dialysis at millimolar concentrations, effectively inhibits only when a weak Mg(2+) chelator is present in the pipette solution. Thus, MgATP acts as a source of Mg(2+) rather than a source of ATP. Using an externally accessible site within the pore of the MIC channel itself as a bioassay, we show that equimolar MgCl(2) and MgATP solutions contain similar amounts of free Mg(2+), explaining the fact that numeric values of Mg(2+) and MgATP concentrations necessary for complete inhibition are the same. Furthermore, we demonstrate that Mg(2+) is not unique in its inhibitory action, as Ba(2+), Sr(2+), Zn(2+), and Mn(2+) can substitute for Mg(2+), causing complete inhibition. We conclude that MIC current inhibition occurs simply by divalent cations.