Rat neocortical neurons grown in dissociated cell culture for 4-12 weeks were studied with whole-cell patch-clamp techniques in order to characterize the calcium currents present in these cells. When voltage-dependent Na and K currents were inhibited, depolarizations from negative holding potentials induced inward currents which had 3 components: a low threshold activated, small, relatively persistent component, which was completely inactivated at holding potentials more positive then -60 mV; a higher threshold, relatively persistent component (which was not inactivated at VH = -50 mV); and a higher threshold, larger, transient component. All 3 components were reduced by removal of Ca, and blocked by Cd and Ni at appropriate concentrations. The components were differentially affected by low concentrations of Ni (500 microM), nifedipine (500 microM) and Ba (1.8 mM). Only the first two components were present in very young neurons.