Voltage-gated K⁺ channels perform many functions in integration of synaptic input and action potential (AP) generation. In this study we show that in bitufted interneurones from layer 2/3 of the somatosensory cortex, the height and width of APs recorded at the soma are sensitive to changes in the resting membrane potential, suggesting subthreshold activity of voltage-gated conductances. Attributes of K⁺ currents examined in nucleated patches revealed a fast subthreshold-inactivating K⁺ conductance (K_f) and a slow suprathreshold-inactivating K⁺ conductance (K_s). Simulations of these K⁺ conductances, incorporated into a Hodgkin-Huxley-type model, suggested that during a single AP or during low frequency trains of APs subthreshold inactivation of K_f was the primary modulator of AP shape, whereas during trains of APs the shape was governed to a larger degree by K_s resulting in the generation of smaller and broader APs. Utilizing the facilitating function of unitary pyramidal-to-bitufted cell synaptic transmission, single back-propagating APs were initiated in a bitufted interneurone by repeated stimulation of a presynaptic pyramidal cell. Ca²⁺ imaging and dendritic whole-cell recordings revealed that modulation of APs, which also affect the shape of back-propagating APs, resulted in a change in dendritic Ca²⁺ influx. Compartmental simulation of the back-propagating AP suggested a mechanism for the modulation of the back-propagating AP height and width by subthreshold activation of K_f. We speculate that this signal may modulate retrograde GABA release and consequently depression of synaptic efficacy of excitatory input from neighbouring pyramidal neurons.