Membrane currents associated with voltage clamp of the giant muscle fibers of a barnacle, Balanus nubilus, were analyzed in terms of currents of the Ca and K channels. Although the activation of the K channel occurs more slowly than that of the Ca channel, both currents show a significant temporal overlap. The currents carried by Ca ++, Sr ++, or Ba ++ through the Ca channel were compared under the conditions at which this overlap was the least. When only one divalent cation is present in the solution, Ba ++ carries more current than Ca ++ or Sr ++ and the sequence of the current is Ba > Sr Ca. When the external solution contains a relatively high concentration of Co ++, which is a blocking agent for the Ca channel, inversion of the sequence occurs, to Ca > Sr > Ba. This is due to the fact that the blocking effect differs depending on which ion carries current through the Ca channel. The Ba current is most sensitive and the Ca current is least affected. Ba suppresses the current of the K channel, independently of its current carrying function through the Ca channel.