1. Whole cell patch-clamp techniques, combined with direct visualization of neurons, were used to study voltage-dependent potassium currents in layer I neurons and layer II/III pyramidal cells. 2. In the presence of tetrodotoxin, step depolarizations evoked an outward current. This current had a complex waveform and appeared to be a composite of early and late components. The early peak of the composite K+ outward current was larger in layer I neurons. 3. In both layer I and pyramidal cells, the composite outward K+ current could be separated into two components based on kinetic and pharmacological properties. The early component was termed I((A)) because it was a transient outward current activating rapidly and then decaying. I((A)) was more sensitive to blocking by 4-aminopyridine (4-AP) than tetraethylammonium (TEA). The second component, termed the delayed rectifier or I((DR)), activated relatively slowly and did not decay significantly during a 200-ms test pulse. I((DR)) was insensitive to blocking by 4-AP at concentrations up to 4 mM and blocked by >60% by 40-60 mM TEA. 4. I((A)) kinetics were examined in the presence of 40-60 mM TEA. Under these conditions, I((A)) began to activate between -40 and -30 mV. Half-maximal activation occurred around 0 mV. In both layer I and pyramidal cells, the half-inactivation potential (V(h-inact)) was around or more positive than - 50 mV. At -60 mV, >70% of I((A)) conductance was available. I((A)) decayed along a single exponential time course with a time constant of ~15 ms. This decay showed little voltage dependence. 5. In both layer I and pyramidal cells, I((DR)) was studied in the presence of 4 mM 4-AP to block I((A)) and in saline containing 0.2 mM Ca2+ and 3.6 mM Mg2+ to reduce contributions from Ca2-+dependent K+ currents. Under these conditions, I((DR)), began to activate at -35 to -25 mV with V(h-act) of 3.6 ± 4.5 mV (mean ± SD). The 10-90% rise time of I((DR)) was 15 ms at 30 mV. At 2.2 ms after the onset of the command potential to +30 mV, I((DR)) could reach a significant amplitude (~1.5 nA in layer I neurons and 2.2 nA in pyramidal cells depending on the cell size). When long test pulses (≤1,000 ms) were used, a decay time constant ~800 ms at +40 mV was observed. In both layer I and pyramidal cells, steady state inactivation of I((DR)) was minimal. 6. These results indicate that I((A)) and I((DR)) are the two major hyperpolarizing currents in layer I and pyramidal cells. The kinetics and pharmacological properties of I((A)) and I((DR)) were not significantly different in fast-spiking layer I neurons and regular-spiking layer II/III pyramidal cells. The relatively positive activation threshold (more than or equal to -40 mV) of both I((A)) and I((DR)) suggest that they do not play a role in neuronal behavior below action potential (AP) threshold and that their properties are more suitable to repolarize AP. The greater density of I((A)) in layer I neurons appears responsible for fast spike generation.