The immunohistochemical distribution of serotonin neurons in normal and transected spinal cords of rats was examined. Intraspinal serotonin neurons were immunostained as far rostral and caudal as T3 and Co1, respectively. All serotonin neurons were located in lamina VII and X, and most were located in spinal autonomic areas. Both bipolar and multipolar neurons were observed with many of the neurons oriented longitudinally to the long axis of the cord. Spinal neurons immunostained for serotonin were visible with and without L-tryptophan and monoamine oxidase inhibitor pretreatment.