1. Whole cell patch-clamp techniques, combined with direct visualization of neurons, were used to study action potential (AP) and repetitive firing properties of layer I neurons in slices of rat neocortex. 2. Layer I neurons had resting membrane potentials (RMP) of -59.8 ± 4.7 mV (mean ± SD) and input resistances (RN) of 592 ± 284 MΩ. Layer II/III pyramidal neurons had RMPs and RNs of -61.5 ± 5.6 mV and 320 ± 113 MΩ, respectively. A double exponential function was needed to describe the charging curves of both neuron types. In layer I neurons, τ0 was 45 ± 22 ms and τ1 was 5 ± 3.3 ms whereas in layer II/III pyramidal neurons, τ0 was 41 ± 11 ms and τ1 was 3 ± 2.6 ms. Estimates of specific membrane resistance (R(m)) for layer I and layer II/III cells were 45 ± 22 and 41 ± 11 kΩcm2, respectively (C(m) was assumed to be 1 μF/cm2). 3. AP threshold was -41 ± 2 mV in layer I neurons. Spike amplitudes, measured from threshold to peak, were 90.6 ± 7.7 mV. AP durations, measured both at the base and half maximal amplitude, were 2.5 ± 0.4 and 1.1 ± 0.2 ms, respectively. AP 1090% rise and repolarization times were 0.6 ± 0.1 and 1.1 ± 0.2 ms, respectively. In layer II/III pyramidal neurons, AP threshold was -41 ± 2.5 mV and spike amplitude was 97 ± 9.7 mV. AP duration at base and half maximal amplitude was 5.4 ± 1.1 ms and 1.8 ± 0.2 ms, respectively. AP 10-90% rise and decay times were 0.6 ± 0.1 ms and 2.8 ± 0.6 ms, respectively. 4. Layer I neurons were fast spiking cells that showed little frequency adaptation, a large fast afterhyperpolarization (fAHP), and no slow afterhyperpolarization (sAHP). Some cells had a medium afterhyperpolarization (mAHP) and a slow afterdepolarization (sADP). All pyramidal cells in layer II/III and 'atypical' pyramidal neurons in upper layer II showed regular spiking behavior, prominent frequency adaptation, and marked sAHPs. 5. In both layer I neurons and layer II/III pyramidal neurons, changes in membrane potential did not greatly alter AP properties. The duration of APs evoked from -50 to - 60 mV was only slightly longer, from -80 to -90 mV. The latency to first spike also was not solely dependent on membrane potential. 6. During repetitive firing, APs broadened in both layer I neurons and layer II/III pyramidal neurons. This was most prominent in pyramidal cells. Broadening was dependent on spike frequency and appeared to result from partial inactivation of both outward potassium and inward sodium currents. 7. In layer I neurons, removing Ca2+ from the bathing solution slightly prolonged spike duration and modestly increased AP firing frequency. These results indicate minimal involvement of Ca2+-dependent K+ currents in AP repolarization. fAHPs were reduced whereas sADPs were abolished. In layer II/III pyramidal neurons, removing Ca2+ reduced or blocked mAHPs and sAHPs and decreased or abolished frequency adaptation. 8. Low concentrations (50 μM) of 4-aminopyridine (4- AP) prolonged APs and induced burst-like firing in layer I neurons. In the presence of 4-AP, the spiking behavior of layer I neurons resembled that of regular spiking layer II/III pyramidal cells. At high concentrations (4 mM), 4-AP could induce a delayed depolarization (DD) after each spike in layer I neurons and in a minority of pyramidal neurons. 9. All layer I neurons had a prominent fAHP that was absent or very small in layer II/III pyramidal neurons. fAHP amplitude was inversely related to AP duration. The reduction of fAHPs by 4-AP or during repetitive firing was accompanied by AP prolongation, suggesting that the current underlying fAHP played an essential role in AP repolarization. The fAHP of layer I neurons could be effectively blocked by 4-AP but only slightly reduced by removing Ca2+ from bathing solution, indicating that the fAHP was mediated primarily by a voltage- dependent transient outward current. 10. Tetraethylammonium (TEA) had similar effects in fast-spiking layer I neurons and regular-spiking pyramidal cells. In the presence of 40-60 mM TEA, APs were increased in duration, often outlasting the current pulse. TEA also produced membrane depolarization, increased synaptic activity, and an increase of RN. 11. These results indicate that layer I neurons display fast-spiking behavior typical of cortical interneurons. Fast APs result from rapid repolarization associated with a fAHP generated primarily by a 4-AP-sensitive current.