The development of spinal cord circuitry in humans is poorly characterized, primarily because standard anatomical tracers must be actively transported, which requires living tissue. Intensely fluorescent lipid-soluble tracers have largely eliminated this problem, at least for circuits that can be traced over short distances. We have, therefore, used the carbocyanine dye DiI (1,1-dioctadecyl-3,3,3,3,-tetramethyl-indocarbocyanine perchlorate) to study the development of the dorsal root afferent projection to fetal human spinal cord between 8 and 19 weeks of gestation. We show here that the dorsal root afferents enter the gray matter of the spinal cord very early in gestation. By 8 weeks, a few axons have already reached the motor pools. These axons, presumably spindle afferents, traverse the length of the spinal gray matter in fascicles to reach different groups of motor neurons. As development progresses, these axons project to the ventral horn and branch in a restricted area in the intermediate zone as well as in the motor pools. Between 11 and 19 weeks of gestation, axons in the ventral horn elaborate boutons that appear to be in proximity to the motor neuron somata and their proximal dendrites. Other groups of axons penetrate the gray matter of the spinal cord all along the mediolateral extent of the dorsal horn. These axons descend to lamina IV, and then turn upward to terminate in laminae III and IV, arborizing primarily rostrocaudally. The time course of the development of these axons parallels that of the axons projecting to the ventral horn.(ABSTRACT TRUNCATED AT 250 WORDS)