Injection of Mg2+ into spinal motoneurons of cats leads to a depolarization, associated with a fall in membrane conductance, diminution in post-spike hyperpolarization, and increased excitability. This action has an apparent reversal level substantially more negative than the resting potential, and can be ascribed to a fall in K+ membrane conductance. Since these effects are opposite to those produced by intracellular Ca2+, it is suggested that Mg2+ probably competes with Ca2+ at the Ca2+-activated K+ ionophoreal free ionophores. Neuronal excitability can be regulated by the ratio of internal free Ca2+/Mg2+.