The effect of the application to the skin of the chemical irritant mustard oil on the size and responsiveness of the cutaneous mechanoreceptive fields of 32 lumbar dorsal horn neurons has been examined in the adult decerebrate, spinal rat. Mustard oil placed on a small region of skin outside the mechanoreceptive firing zone produced a brief (185 +/- 35 sec, SEM) discharge of action potentials in 17 neurons and, in 23 cells, a prolonged increase of the response to a standard low- or high-intensity mechanical stimulus applied to the firing zone of the receptive field. This increase was shown, in 6 intracellularly recorded cells, to be due to a significantly increased depolarization in response to the stimuli. An expansion of the mechanoreceptive firing zones that peaked at 26 +/- 3.7 min was seen in 21 cells. While 6 of 8 nociceptive-specific neurons and 11 of 18 multireceptive neurons showed such an expansion, it did not occur in the 6 cells with low-threshold-only receptive fields. The expansion of the firing zones in 4 intracellularly recorded cells was found to be due to an increased amplitude of the EPSPs evoked by stimuli applied to what had initially been low probability firing fringes (Woolf and King, 1989) outside the firing zones, so that subthreshold responses became suprathreshold after application of the mustard oil. In 4 of 8 nociceptive-specific cells, the mechanical threshold in the firing zone became reduced to innocuous levels after application of the mustard oil. The demonstration of the capacity of a relatively brief afferent barrage of chemosensitive nociceptors to produce an increase in the spatial extent of the cutaneous receptive fields of dorsal horn neurons, amplify their responsiveness, and reduce their thresholds has implications both for the pathogenesis of postinjury pain hypersensitivity phenomena and for receptive field plasticity in the somatosensory system.