5-Hydroxytryptamine (5HT) immunoreactive fibers and varicosities are present in the gray and white matters of the adult domestic fowl spinal cord. These immunoreactive structures are densest in laminae I and II, the area around the central canal, and in the ventral horn. 5HT fibers and varicosities surround certain laminae I and II cells and large ventral horn cells. The apparent intimate relationship between dorsal horn cells and numerous 5HT structures may render them good models to study the possible role of 5HT in the modulation of nociception in the dorsal horn.