The intracellular perfusion technique rendered isolated neurons a convenient object for studying the Ca channels securing inflow of these ions across the membrane into the cell in excitation. Extensive research showed the Ca channels to pass the cations in the sequence Ba greater than Sr greater than Ca greater than Mg and bind these ions with the aid of a binding compound within the channel. Other cation (Co, Ni, Mn, Cd) which are too closely bound with this compound become competitive blocking agents for the channels. In absence of the bivalent cations in the extracellular space the Ca channels loose their selectivity and allow monovalent cations to pass, the reason for this transformation beeing a disruption between the bound Ca ions and the specific regulating compound within the Ca channels. The channels can exist in two states: conducting and nonconducting. The transition from one state to another is accompanied by an intramembrane transfer of charges ("the gate current"). The statistic kinetics of this transition can be described by means of a modified Hodgkin-Huxley equation. In prolonged depolarization the Ca channels become inactivated due to recurrent blocking effect of the Ca ions penetrating in the cell. Some types of the Ca channels, however, reveal a potential-dependent inactivation analogous to that of the sodium or potassium channels.