1. The properties of voltage-gated potassium currents were studied in acutely isolated rat hippocampal pyramidal cells from area CA1 and CA3 at postnatal ages of day 6-8, 9-14, and 26-29 (P6-8, P9-14, and P26-29) with the use of the whole cell version of the patch-clamp technique. 2. The outward current pattern of all cells under investigation could be separated in a fast transient A current (IA) and a delayed rectifier-like current (IK). 3. In both preparations, IA activated and inactivated rapidly. Vh describing steady-state inactivation was -84.5 mV in CA3 cells and -85.5 mV in CA1 cells. The activation behavior was characterized by Vh = -23.8 mV in CA3 cells and -27.2 mV in CA1 cells. The removal of inactivation was monoexponential both in CA1 and CA3 neurons with time constants of 32.1 and 28.5 ms, respectively. IA was insensitive to tetraethylammonium (TEA), dendrotoxin (300 nM), and mast cell degranulating peptide (200 nM), but could be blocked with 5 mM 4-aminopyridine (4-AP) by approximately 80%. In both preparations, A currents did not depend on Ca2+ influx. 4. Delayed rectifier currents (IK) in CA1 and CA3 pyramidal neurons decayed along a double exponential time course. Steady-state inactivation was described by Vh = -79.5 mV in CA3 cells and -76.0 mV in CA1 cells. The activation curves were characterized by midpoints of -3.8 mV in CA3 cells and of -1.4 mV in CA1 cells. The removal of inactivation was monoexponential in CA1 and CA3 neurons with time constants of 210.3 and 202.4 ms, respectively. All kinetic properties were identical for the differentially decaying components of IK. In CA1 cells IK was blocked by TEA at +30 mV with an IC50 of 0.98 mM. In CA3 cells the corresponding IC50 value was 1.05 mM. About 20% of IK were insensitive to TEA. IK was partially blocked by approximately 30% with 100 microM 4-AP. Mast cell degranulating peptide (100-200 nM) and dendrotoxin (50-300 nM) had no effect on IK. 6. Perfusion of charybdotoxin (30 nM), Cd2+ (300 microM), La3+ (10 microM), or Ca(2+)-free solutions resulted in the isolation of a small noninactivating outward current component. Around 10% of IK appeared to be Ca2+ dependent in CA1 neurons. In CA3 pyramidal cells Ca(2+)-dependent outward currents seemed to be somewhat larger with approximately 20%. 7. In CA1 as well as in CA3 cells, the kinetic and pharmacological properties of IA and IK remained stable during postnatal development. However, the contribution of IA and IK to the whole cell current varied with age. IA was more prominent in CA1 cells of age group P6-8 than in age-matched CA3 cells. CA3 cells had smaller A currents and larger delayed rectifier currents than CA1 pyramidal cells. Current densities of IA and IK were analyzed during development to assess changes in the expression of these currents. With increasing postnatal age, the expression of IA was downregulated in both preparations. This effect was more pronounced in CA3 than in CA1 cells. In contrast, IK was upregulated during the same developmental period. This increase in the expression of IK was with approximately 300% much larger in CA1 cells than in CA3 cells with only approximately 50%.