Present concepts of excitable membrane function are based primarily on knowledge of two classes of channels: those activated by neurotransmitters¹ and those activated by membrane potential². Recent evidence suggests that these notions may have to be modified to include other channel types, such as special ion channels activated by membrane potential but regulated by ligands^3–5. We report here studies on single channel currents recorded from heart muscle cells, in which we have found a channel, abundant in cardiac membrane, which does not seem to belong in any of the familiar categories. This channel shows little selectivity between cations, but excludes anions. It is activated by intracellular Ca ions but is not appreciably affected by membrane potential.