1. Patch-clamp techniques were used to record from acutely dissociated rat hippocampal pyramidal cells of different postnatal ages to study the development, kinetics of activation and inactivation, and pharmacology of various components of whole-cell calcium current. 2. In both adult and immature pyramidal cells, the threshold of activation for Ca2+ current from the holding potential of -50 mV was about -35 mV. The current was non-inactivating near threshold, and slowly inactivating with stronger depolarizations. 3. In adult pyramidal cells, hyperpolarizing pre-pulses (-85 mV, 3 s) increased the peak amplitude of current, but had little effect on the amplitude of sustained current or on the threshold. In immature cells, hyperpolarizing pre-pulses (-85 mV, 3 s) revealed an additional component of Ca2+ current that had a threshold for activation around -60 mV, and inactivated rapidly and completely at potentials between -60 and -35 mV. This low-threshold Ca2+ current was found in all cells less than 12 days of age, and in no cells older than 29 days of age. 4. No difference was observed between the Ca2+ currents elicited from CA3 or CA1 pyramidal cells. 5. The time course of decay for inactivating Ca2+ currents in adult cells at -20 mV was well fit with a single exponential of roughly 120 ms time constant. In immature cells, the addition of a second, faster time constant (roughly 25 ms) was required to describe the decay of the inactivating current adequately. The persistent Ca2+ current elicited from the holding potential of -50 mV decayed with a time constant of roughly 750 ms. The rate of inactivation for all Ca2+ current components was faster with stronger depolarizations. Inactivating Ca2+ currents contributed a significantly larger percentage of the total Ca2+ current in immature than adult cells. 6. Steady-state inactivation of the low-threshold Ca2+ current in immature cells was described by the Boltzmann equation with half maximal inactivation at -76 mV and a slope factor of 6.8. Recovery from inactivation was exponential, with a time constant of 820 ms (at -85 mV). Steady-state inactivation of the high-threshold inactivating Ca2+ current in adult cells was described by the Boltzmann equation with half maximal inactivation at -78 mV and a slope factor of 9.9. 7. The low-threshold Ca2+ current in immature cells was blocked in a reversible and dose-dependent manner by amiloride (100-250 microM). Amiloride had no effect on high-threshold Ca2+ currents.(ABSTRACT TRUNCATED AT 400 WORDS)