Nociceptors in the C or Aδ conduction velocity range are likely to mediate slow and fast components of pain respectively. The aim of the present study was to investigate the properties of spinal neurones that are selectively activated by these two classes of afferent. Four Wistar rats were anaesthetised (pentobarbitone 31 mg kg^-1 h^-1 i.v.) and slow (2.5°C s^-1; n=2) or fast (7.5°C s^-1; n=2) ramps (30-55 or 57°C, respectively) of contact heat applied (six times in each animal) to the dorsal surface of the left hind paw to preferentially activate C- or Aδ- nociceptors, respectively (McMullan et al. 2004). Two hours later, anaesthesia was deepened and the animals were perfusion fixed. Transverse sections were cut from midlumbar segments and reacted immunochemically to visualise Fos protein in cells activated by the heat stimuli. Both types of stimuli activated clusters of cells in laminae I-II of the ipsilateral mid-dorsal horn. Sections containing Fos were reacted immunochemically for neurochemical markers that are associated with either excitatory (the neurokinin-1 receptor (NK-1), protein kinase Cγ (PKCγ), calbindin (Cal), the μ opioid receptor 1 (MOR-1)) or inhibitory (nitric oxide synthase, parvalbumin and choline acetyltransferase) spinal cord neurons (Todd & Spike, 1993; Kemp et al. 1996). A quantitative confocal microscopic study was performed to determine which of the markers was associated with Fos. None of the markers for inhibitory cells contained Fos but it was present in cells labelled with all the markers for excitatory cells in both fast (F) and slow (S) ramp experiments (NK-1, F=31% (32/102 Fos cells), S=21% (83/398 Fos cells); PKCγ, F = 23% (24/102 Fos cells), S=15% (59/398 Fos cells); Cal, F=28% (107/382 Fos cells), S=24% (204/849 Fos cells); MOR-1, F=10% (28/292 Fos cells), S=7% (62/909 Fos cells)). The results suggest that both C and Aδ fibres activate excitatory neurons in the dorsal horn which are likely to include interneurons and projection cells. Further studies are in progress to determine if the number of cells activated by the two rates of skin heating differ significantly.