A recent study described upregulation of voltage gated Kv3.4 potassium channels in early and late stages of Alzheimer’s disease (Angulo et al. 2004). However, little is known about their distibution and functional roles in central neurones. We therefore sought to determine the localisation of channels containing Kv3.4 subunits in the hippocampal formation. Using immunohistochemistry, we find that of the Kv3 subfamily (Kv3.1-4) only Kv3.4 subunits are expressed in CA1 and CA3 pyramidal cells and present both pre- and post-synaptically at presumptive Schaffer collateral synapses on CA1 pyramidal cells. To investigate the functional role of channels containing Kv3.4 subunits we performed whole-cell patch clamp recordings from CA1 pyramidal cells in 300 μm hippocampal brain slices obtained from 12 day old rats (data are presented as mean ± S.E.M., where n=number of cells). Voltage steps from -90 mV to +40 mV produced rapidly activating outward potassium currents with a large transient component in CA1 pyramidal cells. Application of the Kv3.4 subunit toxin blood depressing substance (BDS-II) at 50-100 nM significantly reduced the transient component of the outward current (peak current density reduced from 207 ± 36 to 72 ± 30 pA/pF; n=5; P<0.05 Student's T test) and shifted the V_1/2 from -15 ± 3 mV to -23 ± 2 mV (single Boltzmann function, n=5). We also have preliminary evidence that blocking Kv3.4 channel subunits increases action potential duration in CA1 pyramidal cells and alters the amplitude of evoked EPSCs. Our data suggest a role for channels containing Kv3.4 subunits in regulating intrinsic excitability and synaptic transmission in hippocampal neurones.