Kv3 channels are voltage-gated potassium ion channels and are highly expressed in the brain. Here, they are important in neuronal firing and synaptic transmission (Rudy et al., 1999). However, Kv3 channel subunit expression has been found in some areas to decrease with age with functional consequences (Zettel et al., 2007). Kv3 channels are also expressed in the spinal cord (Deuchars et al., 2001) but little is known about their relevance to the functioning of spinal circuitry and whether age-related changes are observed here. In this presentation, we identify co-localisation of Kv3 subunits, Kv3.1b and Kv3.3, with both excitatory and inhibitory synaptic markers around putative bladder motoneurones within the spinal cord and show that levels of Kv3 immunoreactivity here also decrease with age. Young (n=3) and aged (n=3) C57bl6 mice were anaesthetised intraperitoneally with 60 mg/kg pentobarbitone, perfused transcardially and fixed with 4% paraformaldehyde. Lumbosacral spinal levels (L1, L6 and S1) were dissected, sectioned to 20 µm and processed for double labelling immunohistochemistry of Kv3 subunits Kv3.1b and Kv3.3 with inhibitory synaptic markers, VGAT and GlyT2, and excitatory synaptic marker, VGluT2. Immunohistochemical analysis revealed punctate immunoreactivity for Kv3.1b and Kv3.3 subunits around somata of putative bladder motoneurones; namely parasympathetic preganglionic neurones (PGN), sympathetic preganglionic neurones (SPN) and dorso-lateral nucleus (DLN) neurones in the lumbo-sacral spinal cord. Kv3 puncta around these motoneurones co-localised with both excitatory and inhibitory synaptic immunoreactivity. In a comparison of young and aged mice, the number of Kv3 puncta around DLN, PGN and SPN motoneurones was significantly reduced (DLN, Kv3.3; Ind. Equ. T-test, young, 57.67 ± 16.5 vs aged, 49.26 ± 14.3, p<0.01, PGN, Kv3.3; Ind. Equ. T-test, young, 67.18± 14.8 vs aged, 43.8 ± 15.47, p<0.001, SPN, Kv3.1b; Ind. Unequ. T-test, young, 124.4 ± 27.0 vs aged, 87.7 ± 36.5, p<0.001. Data as Mean ± SEM) and changes in the degree of co-localisation were also observed. We hypothesise that the co-localisation of Kv3 subunits with synaptic immunoreactivity around putative bladder motoneurones, and a decline in expression here in aged mice, may have functional significance within bladder circuitry and on the bladder reflex with age. Future work, therefore, aims to characterise Kv3 currents of neurones at this level and establish a role for Kv3 channels within this circuitry.