SUMMARY1. Ca2+ inward currents evoked by membrane depolarization have been studied by the intracellular dialysis technique in the somatic membrane of isolated dorsal root ganglion neurones of new-born rats. In about 20 % of the investigated cells a hump has been detected on the descending branch of the current-voltage curve, indicating the presence of two populations of Ca2+ channels differing in their potential-dependent characteristics.2. An initial less regular component ofthe Ca2+ current was activated at membrane potentials from -75 to -70 mV. Its amplitude reached 02-{09 nA at 14-6 mMextracellular Ca2+. The activation kinetics of this component could be approximated by the Hodgkin-Huxley equation using the square of the m variable. Tm varied in the range from 8 to 1 ms at potentials between -60 and -25 mV ('fast' Ca2+ current).3. The second component of the Ca2+ current was activated at membrane depolarizations to between -55 and -50 mV. It could be recorded in all cells investigated and reached a maximum value of 1-7 nA at the same extracellular Ca2+ concentration. This component decreased rapidly during cell dialysis with saline solutions. The decrease could be slowed down by cooling and accelerated by warming the extracellular solution. Intracellular introduction of 3',5'-cAMP together with ATP and Mg2+ not only prevented the decrease but often restored the maximal current amplitude to its initial level. The activation kinetics of this component could also be approximated by a square function, Tm being in the range 16-2-5 ms at membrane potentials between -20 and + 3 mV ('slow' Ca2+ current).4. The fast Ca2+ current inactivated exponentially at sustained depolarizations in a potential-dependent manner, Th varying from 76 to 35 ms at potentials between -50 and -30 mV. The inactivation of the slow Ca2+ current studied in double-pulse experiments was current-dependent and developed very slowly (time constant of several hundreds of milliseconds). It slowed down even more at low temperature or after substitution of Ba2+ for Ca2+ in the extracellular solution. 5. Both currents could also be carried by Ba2+ and Sr2+, although the ion-selecting properties of the two types of channels showed quantitative differences.

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