Whole cell patch recordings were obtained from medium diameter (35–45 μm) dorsal root ganglion neurons. Using electrophysiological parameters, we were able to subclassify acutely dissociated dorsal root ganglion cells into three uniform (types 5, 6 and 9) and one mixed class (type 8) of neurons. All subtypes (types 5, 6, 8 and 9) had broad action potentials (7.0±0.2, 5.2±0.4, 7.3±0.5 and 6.0±0.4 ms) and exceptionally long afterhyperpolarizations (112±9, 178±19, 124±31 and 204±33 ms). Long afterhyperpolarizations have been linked to mechanically insensitive (silent) nociceptors by other laboratories [Djouhri et al., J. Physiol. 513 (1998) 857–872]. Chemosensitivity varied among cell classes. Cell types 5, 8 and 9 were capsaicin sensitive (45±13, 87±30 and 28±13 pA/pF; 5 μM) groups, while the type 6 cell was capsaicin insensitive. All cell types expressed ASIC-like (acid sensing ion channel) amiloride sensitive, proton-activated currents with a threshold of pH 6.8 and a peak near pH 5.0. All medium sized cells were sensitive to ATP (50 μM) and exhibited the ‘mixed’ form of ATP-gated current [Burgard et al., J. Neurophysiol. 82 (1999) 1590–1598; Grubb and Evans, Eur. J. Neurosci. 11 (1999) 149–154]. Immunohistochemistry performed on individual cells indicated the expression of both P2X₁ and P2X₃ subunits. Electrophysiologically defined classes were histochemically uniform. All types were examined for the presence of substance P (SP), calcitonin gene related peptide (CGRP) and binding of isolectin B4 (IB4). All subtypes expressed CGRP immunoreactivity. Types 5 and 8 co-expressed SP and CGRP immunoreactivity and also bound IB4. Subtypes 6 and 9 were positive for neurofilament m. It is likely that these cells represent major classes of myelinated and unmyelinated peptide expressing nociceptors.