A depolarization-induced, slowly decaying inward current was examined in slice-cultured CA3 pyramidal cells by voltage-clamp techniques and microfluorometric measurements of cytosolic free Ca2+ concentration ([Ca2+]i). Action potentials elicited by intracellular injection of short-lasting (50-100 ms) depolarizing current pulses were followed by a slowly decaying afterhyperpolarization (AHP). After switching to voltage-clamp mode, short-lasting (50-100 ms) depolarizing voltage jumps from -60 mV to between -30 and 0 mV induced a slowly decaying outward aftercurrent (IAHP) which was depressed by bath application of muscarine (0.5 microM). In the presence of muscarine, the same depolarizations induced a slowly decaying afterdepolarization (ADP) or inward aftercurrent (IADP) in voltage-clamp mode. This current was also induced in the presence of trans(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid (t-ACPD, 5 microM), an agonist of metabotropic glutamate receptors, but not in the presence of noradrenalin (5 microM), while both of these agonists depressed IAHP. IADP was depressed by reducing the external Ca2+ concentration from 3.8 to 0.5 mM, by external Co2+ (1 mM) and by external Cd2+ (10-100 microM). Combined voltage-clamp recordings and microfluorometric measurements of [Ca2+]i using the Ca2+ indicator fura-2 revealed that the amplitude of IADP was correlated with the amplitude of depolarization-induced Ca2+ influx. IADP was absent at membrane potentials < -90 mV, and reached maximal amplitudes at approximately -55 mV. Raising the extracellular K+ concentration from 2.7 to 13.5 mM increased the amplitude of IADP and resulted in a positively directed shift of the apparent reversal potential of IADP. When the external Na+ concentration was reduced from 157 to 33 or 18 mM the current reversed at more negative potentials and was reduced to 40 and 21%, respectively, of control amplitude. Lowering the external CI- concentration from 159 to 20 mM did not affect IADP. We conclude that IADP most likely represents a Ca(2+)-activated cation current, rather than a Ca2+ tail current, or an electrogenic Ca2+ extrusion current.