Experiments were carried out in pentobarbitone-anaethetised adult rats (31mg kg^-1 h^-1) to determine the effects of neuronal activation in the ventrolateral PAG on the numbers and laminar distributions of spinal neurones activated by C- or Aδ-heat nociceptors. ‘Slow’ (2.5°C s^-1) or ‘fast’ (7.5°C s^-1) ramps (30-55 or -57°C, respectively) of contact heat were applied (6 times in each animal) to the hairy skin of the dorsal surface of the left hind paw to preferentially activate C- or Aδ-nociceptors, respectively (McMullan et al. 2004). A glass pipette was positioned at sites in the PAG at which injection of DL-homocysteic acid (30nl, 50mM) prior to the heat stimulus, generally evoked depressor responses as an indication of activation in the VL-PAG. Rats were assigned to 1 of 6 groups: two control groups, one unstimulated (n=4) and one stimulated in the PAG alone (n=2): four experimental groups, two in which slow rates of skin heating were used, one in the absence of VL-PAG stimulation (n=6) and one following VL-PAG activation (n=2); and two groups in which fast rates of skin heating were used, one in the absence of PAG stimulation (n=8) and one following VL-PAG activation (n=4). Two hours later anaesthesia was deepened and the animals were perfusion fixed. 50μm sections of the PAG were processed to visualise dye injection sites. 40μm sections of the lumbar (L3-6) spinal cord were processed immunocytochemically to visualise neurones in which Fos protein was evoked by the heat stimulus. Ten sections were selected for analysis in each control and experimental animal. Activation in the VL-PAG reduced the numbers of Fos-positive neurons in laminae I and II in response to slow rates of skin heating by 44% and 61%, respectively. Following fast rates of skin heating the number of Fos-positive neurons was reduced by 37% in lamina I and increased by 25% in lamina II. The approach used has provided a quantitative measure of the effects of stimulation in the ventrolateral PAG on the numbers and laminar distributions of superficial dorsal horn neurones activated by C- compared with Aδ-nociceptors. The data reveal potential differential control of C- versus Aδ- nociceptive processing in superficial dorsal horn.