Spiral Ganglion Neurons (SGNs) are the first primary afferent neurons of the auditory pathway. Numerous SGNs innervate each sensory inner hair cell of the cochlea and transmit action potentials to the auditory brainstem. In vivo recordings show a complexity in the firing properties between individual SGNs, a feature attributed to variations in pre- and post-synaptic mechanisms. In SGNs cultured from pre-hearing mice, the population shows a heterogeneous distribution of adaptation rates (Mo and Davis, 1997). Rapidly-adapting neurons have a prominent Dendrotoxin-K-sensitive low voltage-activated (LVA) K+ current, implicating voltage-gated K+ (Kv) subunits of the Kv1.1 subtype (Mo et al., 2002). The exact composition of the underlying channel subunits remains undetermined, as does the physiological relevance of these channels in mature hearing. Here, we used a pharmacological approach to investigate the potential contribution of Kv1.2 subunits to the LVA current in post-hearing mice. Primary SGN cultures, prepared from C57BL/6 mice, were maintained for 2-3 days in vitro for whole-cell patch clamp electrophysiology. Two age groups, P12-14 and P20-21 were used to compare the properties of SGNs around hearing onset and one week post-hearing, respectively. Current-clamp recordings revealed the presence of slowly-adapting and rapidly-adapting neurons in both age groups, irrespective of cochlear tonotopic location. Rapidly-adapting neurons were associated with a distinct LVA K+ current. The application of 100 nM Tityustoxin-Kα (TsTx) which blocks channels containing Kv1.2 subunits caused a substantial reduction of the conductance in the LVA region in both age groups (mean ± SEM: P12-14, 66.0 ± 6.5%, n=6; P20-21, 72.2 ± 4.3%, n=6). Similarly, the application of another structurally unrelated Kv1.2-specific blocker, κM-Conotoxin RIIIJ, blocked the LVA conductance in P12-14 SGNs (70.8 ± 7.8%, n=5). The co-application of Dendrotoxin-K, which blocks channels containing Kv1.1 subunits, did not substantially enhance the block by TsTx (P12-14, 66.8 ± 8.9%, n=6; P20-21, 80.2 ± 5.7%, n=6), suggesting that Kv1.1 and Kv1.2 subunits do not exist as two homomeric populations. Consistent with the electrophysiological findings, immunofluorescence localised Kv1.1 and Kv1.2 subunits to the soma and proximal regions of neurites in SGNs in vivo. Together, these findings suggest that heteromeric channels comprising Kv1.1 and Kv1.2 subunits underlie the LVA current in SGNs thereby contributing to rapid adaptation. The intrinsic properties of SGN membrane physiology may therefore contribute to the complexity of observed input/output functions of auditory nerve fibres in vivo.