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Back-propagating action potentials in CA1 pyramidal neurons may provide the postsynaptic dendritic depolarization necessary for the induction of long-term synaptic plasticity. The amplitudes of back-propagating action potentials are not all or none but are limited in amplitude by dendritic A-type K+channels. Previous studies of back-propagating action potentials have suggested that prior depolarization of the dendritic membrane reduces A-type channel availability through inactivation, resulting in an enhanced, or boosted, dendritic action potential. However, inactivation kinetics in the subthreshold potential range have not been directly measured. Furthermore, the corresponding rates of Na+channel inactivation with depolarization have not been considered. Here we report in cell-attached patches (150–220 μm from the soma, 32°C) that at 20-mV positive to rest, A-type K+channels inactivated with a single exponential time constant of 6 ms, whereas Na+channels inactivated with a time constant of 37 ms. The ratio of available Na+to K+current increased as the duration of the depolarization increased. Thus the subthreshold properties of Na+and A-type K+channels provide a mechanism by which information about the level of synaptic activity may be encoded in the amplitude of back-propagating action potentials.