SUMMARY1. Ca2+ currents were measured in single cells isolated from frog ventricle using the whole-cell patch clamp technique and a perfused pipette. K+ currents were blocked with intracellular (120 mM) and extracellular (20 mM) Cs+.2. A single type of Ca2+ current (ICa) was found in these cells. The current activated at voltages positive to -30 mV, exhibited a symmetrical current-voltage relationship with a peak at 0 mV, and was slowly inactivating with Ba2+ as charge carrier.3. Large variations in ICa amplitude were observed from cell to cell (ICa at 0 mV = 29341 + 283-3 pA; N = 152). These variations were not due simply to differences in cell membrane area, which was estimated by cell membrane capacitance (Cm), because the density of Ca2+ current (dIca = Ica/Cm) also varied significantly from cell to cell (1I3-28 pA/pF at 0 mV; mean+ S.D. = 4-49+3-96; N = 152).4. The inactivation curve of ICa was a complex function of mnembrane potential. 200 ms pre-pulses to voltages between -60 and + 20 mV progressively inactivated ICa elicited by a subsequent test pulse with half-maximal inactivation occurring for pre-pulses to -40 mV. With pre-pulses positive to + 20 mV, ICa elicited by the test pulse became progressively larger. The degree of inactivation induced by a 200 ms depolarization to potentials more positive than + 20 mV varied significantly from cell to cell, while no such variations were observed in the negative range of membrane potentials.5. The time course of reactivation (i.e. removal from inactivation) of ICa at -80 mV often exhibited an overshoot. The amplitude of the overshoot varied between 100% (i.e. no overshoot) and z 180% in eighty-one cells.6. The degree of inactivation at positive potentials (+ 100 mV) and the amplitude * Present address:

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