Certain membrane properties of axon-ligated cerebral A neurons in Aplysia californica were examined under voltage-clamp conditions (temperature, 8-13 degrees C). Depolarization was found to induce two kinds of transient inward currents, carried by Na+ and Ca2+ and similar to the Na+ and Ca2+ currents reported in other molluscan nerve cells. These currents were differentiated by introducing specific channel blockers or ion substitutions into the bathing medium. Certain characteristics of the Na+ and Ca2+ currents in A neurons were found to differ from those previously reported for molluscan neurons. The threshold potentials for both Na+ and Ca2+ currents were more negative and more the same than described for other Aplysia neurons. The threshold for Na+ current was -37 +/- 6 mV (mean +/- SD; N = 6). The threshold for Ca2+ current was -40 +/- 1 mV (N = 4). The time course of Na+ inactivation in A neuron soma was found to have two components that could be described as the sum of two exponential processes with time constants th(Na)1 and th(Na)2. Na+ inactivation was voltage dependent, with both time constants for inactivation becoming smaller at positive potentials. The faster components varied from approximately 4 to 1 msec over a range from -30 to +20 mV. The slower component of Na+ decay had a time constant that varied from approximately 9 to 3 msec over a range of -30 to +20 mV. Recovery from complete Na+ inactivation had a delay. Steady-state inactivation of the Na+ conductance was voltage dependent, with a 5-mV change at half-inactivation (-51 mV) producing an e-fold change. Activation of the Na+ current after correction for inactivation could be described by the expression GNa(t) = GNa(infinity) [1 - exp[-t/tm(Na)]]3 X tm(Na) was voltage dependent, varying from approximately 2 to 1 msec over a range of -30 to +20 mV. Inactivation of the Ca2+ currents had two rate components which proceeded at about 1/5 to 1/100 the rate of Na+ inactivation. The time course of Ca2+ inactivation was best described by assuming that it had two exponential components with time constants th(Ca), and th(Ca)2. Both time constants were voltage dependent. The larger time constants, th(Ca)2, changed from approximately 1.3 to 0.4 sec over a range of -30 to +20 mV X th(Ca)1 changed from approximately 130 to 40 msec over a range of -30 to +20 mV. Steady-state inactivation of the Ca2+ conductance was voltage dependent, with a 5-mV change at half-inactivation (-44 mV) producing an e-fold change.(ABSTRACT TRUNCATED AT 400 WORDS)