Voltage-gated potassium channels effectively regulate dendritic excitability in neurones. In the distal apical dendrite of layer 5B (L5B) neocortical pyramidal neurones, potassium conductances are suggested to participate in active dendritic synaptic integration and control regenerative dendritic potentials. The ionic mechanism for triggering these regenerative potentials has yet to be elucidated. Here we used two-electrode voltage-clamp (TEVC) to quantitatively record K⁺ conductance densities of a sustained K⁺ conductance in soma and apical dendrite of L5B neurones of adult rats. We report that the somatic and proximal dendritic sustained voltage-gated K⁺ conductance density is more than 10-fold larger than previously estimated and closely matches predictions made by simulations. The results obtained using TEVC were corroborated using cell-attached and current-clamp experiments in combination with compartmental modeling. Possible error sources, including inaccurate measurement of the passive membrane parameters and unknown axonal and basal dendritic conductance distributions, were shown not to distort the density estimation considerably. Finally, the sustained voltage-gated K⁺ conductance density was found to decrease steeply along the apical dendrite. The steep negative K⁺ conductance density gradient along the apical dendrite may help to define a distal, low threshold region for dendritic amplification of distal synaptic input in L5B pyramidal neurones.