Intracellular electrophysiological recordings were made in vitro at 36.5°C from lumbar (L4 to L6) dorsal root ganglion neurones of 6–8‐week‐old female rats. Electrophysiological properties were recorded prior to intracellular injection with fluorescent dye. The following showed substance P‐like immunoreactivity (SP‐LI): 8/19 C‐fibre cells, 6/26 Aδ cells, and 0/52 Aα/β cells. In C‐fibre neurones, there were no significant differences in action potential (AP) characteristics between those with SP‐LI and those without. In contrast, the Aδ neurones with SP‐LI had significantly deeper and longer afterhyperpolarisations (AHPs), but their AP durations did not differ from those without SP‐LI. Both SP‐LI and calcitonin gene‐related peptide‐LI (CGRP‐LI) were examined on 18 Aδ and 11 C cells. Most (7/8) neurones with SP‐LI also showed CGRP‐LI, but only 7/21 neurones without SP‐LI showed CGRP‐LI. One C cell showed SP‐LI but no CGRP‐LI. Neurones with neither peptide (−/−), with only CGRP (−/CGRP) or with both peptides (SP/CGRP) were compared (n = 28). The numbers in each group were, respectively, 5, 2, and 3 with C‐fibres and 9, 5, and 4 with Aδ‐fibres. The peptide content and AP shape were related in Aδ neurones. Most −/− Aδ neurones had short APs and short AHPs; most −/CGRP neurones had long APs and long AHPs, and SP/CGRP neurones had short APs with deep, long AHPs. There was a positive correlation between log₁₀ of the area under the AP (AP area) and log₁₀ AHP duration in Aδ neurones. All SP‐ and most CGRP‐containing Aδ neurones had AP shapes similar to those previously described for nociceptive neurones. However, a few without peptide also showed such properties, raising the possibility that some nociceptive neurones did not express these peptides. J. Comp. Neurol. 388:541–549, 1997. © 1997 Wiley‐Liss, Inc.