To determine the participation of NK-1 receptors in mediating inputs from noxious thermal stimulation, sustained noxious stimulation was applied to anaesthetized rats by immersing one hind paw in water for 1.5 min at different temperatures. After sacrificing the animal, lumbar spinal cords were removed and 20 microns sections were incubated with [125I]BH-substance P. Compared with unstimulated controls, binding was the least in rats given a 55 degrees C stimulus and sacrificed 1 min after the stimulus; the greatest reduction was found in the superficial dorsal horn. Rats sacrificed at 10 min showed intermediate binding levels. Groups given less intense stimuli showed smaller decreases in binding. It is suggested that the decrease in binding was due to occupation of receptors by endogenous ligand, which is consistent with the idea that noxious stimulation evokes the release of substance P at the spinal level.