Voltage-gated potassium channels were studied in cell-attached and outside-out patches from the soma and primary apical dendrite of large layer 5 pyramidal neurons in acute slices of rat sensorimotor cortex (22-25 degrees C). Ensemble averages revealed that some patches contained only fast, I(A)-like channels, other contained only I(K)-like channels that did not inactivate or inactivated slowly, and the remainder contained mixtures of both types. I(A) and I(K) channels had mean unitary conductances of 8.5 and 20.3 pS, respectively, and had distinctive patterns of gating. Peak activation curves for ensemble-averaged currents were described by the Boltzmann equation with half-maximal voltage [V(1/2)] and slope factor (k) values of -24.5 mV and 16.9 mV for I(A) and -7.6 mV and 10.1 mV for I(K) (patches < 250 microm from the soma) or -22.9 mV and 16.2 mV for I(A) (patches > 250 microm from the soma). The steady-state inactivation curve for I(A) gave V(1/2) and k values of -72.3 mV and -5.9 mV (< 250 microm from the soma) or -83.1 mV and -6.5 mV (> 250 microm from the soma). These values were similar to the corresponding data for I(A) and I(K) in nucleated patches from the same cell. The amount of I(A) and I(K) present in patches depended weakly on distance along the primary apical dendrite from the soma. The amplitude of I(A) increased, on the average, by 2.3 pA per 100 microm, while the amplitude of I(K) decreased by 0.4 pA per 100 microm. I(A) and I(K) channels in dendritic cell-attached patches were activated by the passage of a back-propagating action potential past the tip of the patch electrode. These results show directly that these potassium channels participate in action potential repolarisation, and thus contribute to the process of synaptic integration in these neurons.