Calcium channels in excitable membranes are essential for many cellular functions. Recent analyses of the burst-firing mode of some vertebrate neurones suggest that changes in their functional state are controlled by a Ca conductance that is largely inactivated at resting membrane potentials (-50 to -60 mV), but becomes activated following a conditioning hyperpolarization of the cell membrane. Here, using chick and rat sensory neurones, we present evidence for a new type of Ca channel with time- and voltage-dependent properties which is probably responsible for the inactivation behaviour of the Ca conductance. At membrane potentials between -50 and +10 mV, openings of this channel last 3-6 ms and tend to occur in rapid succession. Inactivation of this channel is indicated by prolonged and eventually complete closures brought about by long-lasting depolarizing voltage steps. This channel coexists in isolated membrane patches with the more common Ca channel which is less sensitive to changes in holding potential and shows a considerably shorter average life time and smaller currents.