The α3 isoform of the Na⁺/K⁺ ATPase is selectively expressed in a subpopulation of large diameter primary afferent neurones and in fibres that project to muscle spindles (Dobretsov et al. 2003). In the present study we examined immunocytochemically the distribution of this isoform in dorsal root ganglion (DRG) neurones classified according to sensory receptor properties and conduction velocity in deeply anaesthetised rats as previously described (Lawson et al. 1997). α3-like immunoreactivity was examined with an affinity purified polyclonal antibody, previously fully characterised with Western blotting and causing staining patterns similar to other anti-α3 antibodies. Young adult female Wistar rats were deeply anaesthetized throughout the experiments with sodium pentobarbitone (70-80mg/kg, i.p.) and during recording, regular doses of pancuronium (0.6mg/kg, i.v.) were given always with additional (20mg/kg, i.v.) anaesthetic. End-tidal CO₂ and blood pressure were monitored throughout. Intracellular recordings were made in 81 L4-L6 DRG neurones with dye-filled glass microelectrodes. For each neurone, after identification of sensory and electrophysiological properties, the fluorescent dye was electrophoretically injected into the neuronal soma. Rats were killed with an anaesthetic overdose and perfused with Zamboni’s fixative. DRGs were post-fixed and frozen sections (7 μm) cut. ABC immunostaining for α3 on dye-injected DRG neurones enabled measurement of relative staining intensity (percentage maximum staining in that section). In addition, double-label fluorescence immunocytochemistry was carried out on lumbar DRGs from 4 rats, with antibodies to neurofilament (RT97) and α3. Muscle spindle afferents (MSAs) did not have cutaneous receptive fields, usually showed ongoing firing resulting from stretch of the leg and presence of muscle relaxant, showed altered firing rates with gentle pressure on muscle or altered leg position and followed a rapid vibratory stimulus. Of the Aα/β-fibre neurones examined, all 17 MSAs stained positively for α3, with dense staining over the cell membrane – appearing as a stained ‘ring’ around the cell. Furthermore, in MSAs stained positively for α3, there were significant correlations (P<0.05) between relative ring staining intensity and a) membrane potential magnitude (positive correlation, r²=0.48), and b) action potential fall time and duration at base (negative correlations, r²= 0.38 and 0.52, respectively). In contrast, the ring staining was low (weakly stained or absent) in other Aα/β-fibre groups (nociceptive units and cutaneous low threshold slowly and rapidly adapting units). The absence of detectable ring staining seen in 14/15 slowly conducting afferents studied (i.e. those with Aδ- or C-fibres), was consistent with the lack of clear α3 membrane staining in most neurones with neurofilament-poor cytoplasm. Thus the α3 Na⁺/K⁺ ATPase isoform is highly expressed in MSAs and may contribute to their functional properties.