Immunoreactive staining for calcitonin gene-related peptide (CGRP) in the rat superficial dorsal horn (L4-L5 segments) was examined following electrical stimulation of the sciatic nerve. Electrical stimulation for twenty minutes at an intensity to elicit both A and C fiber volleys resulted in a significant decrease in CGRP immunoreactivity in the medial two-thirds of the dorsal horn on the stimulated side as compared to the non-stimulated side of the spinal cord. The change was most pronounced in the medial area of the dorsal horn, with a 54% decrease in immunostaining density compared to the non-stimulated side. In contrast, when the sciatic nerve was stimulated for the same period at an intensity that stimulates only A fibers, there was no significant difference in the density of immunoreactive staining in the dorsal horn between the non-stimulated and stimulated sides of the spinal cord. These results suggest that activity in a specific population of primary afferent fibers causes a dramatic depletion of CGRP in the dorsal horn.