Voltage-gated conductances like Kv7 channels (mediating subthreshold 'M-current') and voltage-gated sodium channels (mediating subthreshold persistent sodium current) in neurons have been shown to confer both frequency-dependent resonance and filtering of membrane potential oscillations (Hu et al., 2002, 2009), favoring propagation of subthreshold voltage signals in the theta-frequency range (around 5–8 Hz), e.g. from dendrites to the soma. Recent work has shown that subthreshold somato-dendritic voltage fluctuations can be transferred passively into axons and that these signals can modulate action potential-dependent transmitter release (Alle and Geiger, 2006; Shu et al., 2006). Immunohistochemical data indicate that hippocampal mossy fibers contain Kv7 channels (Cooper et al., 2001), and together with voltage-gated sodium channels they might cause frequency-dependent resonance and filtering. Whole-cell recordings from mossy fiber boutons (MFBs) revealed a slowly activating, non-inactivating M-current (27 pA at -60 mV, blocked by 10 uM XE991 and enhanced by 10 mM retigabine), as well as a persistent sodium current (NaP; -11 pA at -60 mV, blocked by TTX). Injection of subthreshold oscillating currents into MFBs with constant amplitude but linear increase in frequency (ZAP) evoked voltage fluctuations with a 'local' resonance peak at around 17 Hz eliminated by XE991. The frequency dependence and maximal input impedance during ZAP application at MFBs was reproduced in a multicompartmental model of a hippocampal granule cell. In this model, application of the ZAP at the soma resulted in maximal voltage fluctuation transfer at theta-frequencies, which occur in granule cell somato-dendritic domains during e.g. explorative behavior of rodents.