Dorsal root ganglia (DRG) neurons synthesize and transport substance P (SP) to the spinal cord where it is released in response to intense noxious somatosensory stimuli. We have shown previously that SP release in vivo causes a rapid and reversible internalization of SP receptors (SPRs) in dorsal horn neurons, which may provide a pharmacologically specific image of neurons activated by SP. Here, we report that noxious heat (43 degrees, 48 degrees, and 55 degrees C) and cold (10 degrees, 0 degrees, -10 degrees, and -20 degrees C) stimuli, but not innocuous warm (38 degrees C) and cold (20 degrees C) stimuli, applied to the hindpaw of anesthetized rats induce SPR internalization in spinal cord neurons that is graded with respect to the intensity of the thermal stimulus. Thus, with increasing stimulus intensities, both the total number of SPR+ lamina I neurons showing SPR internalization and the number of internalized SPR+ endosomes within each SPR immunoreactive neuron showed a significant increase. These data suggest that thermal stimuli induce a graded release of SP from primary afferent terminals and that agonist dependent receptor endocytosis provides evidence of a spatially and pharmacologically unique "neurochemical signature" after specific somatosensory stimuli.