The first objective of the present study was to ask when axons of the fasciculus gracilis reach the nucleus gracilis in the North American opossum (Didelphis virginiana). When Fast Blue (FB) was injected into the lumbar cord on postnatal day (PD) 1 and the pups were killed 2 days later, labeled axons were present within a distinct fasciculus gracilis at thoracic and cervical levels of the cord. When comparable injections were made at PD3 or 5 and the pups were allowed to survive for the same time period, a few labeled axons could be followed to the caudal medulla where they were located dorsal to the presumptive nucleus gracilis. In order to verify these observations and to determine if any of the axons which innervate the nucleus gracilis early in development originate within dorsal root ganglia, we also employed cholera toxin conjugated to horseradish peroxidase (CT-HRP) to label dorsal root axons transganglionically. When CT-HRP was injected into the hindlimb on PD1 and the pups were maintained for 1 day prior to death and HRP histochemistry, labeled axons were present within the fasciculus gracilis at thoracic and cervical levels, but they could not be traced into the medulla. When comparable injections were made on PD3, and the pups were maintained for 2 days, labeled axons were present within the caudal medulla. Our second objective was to determine whether axons of the fasciculus gracilis grow through a lesion of their spinal pathway during early development. In one group of animals, the thoracic cord was transected at PD5, 8, 12, 20 and 26 and bilateral injections of Fast Blue (FB) were made four segments caudal to the lesion 30-40 days later. After a 3-5 day survival, the pups were killed and perfused so that the spinal cord and brainstem could be removed and sectioned for fluorescence microscopy. In all of the cases lesioned at PD5, axons of the fasciculus gracilis were labeled rostral to the site of transection and they could be followed to the nucleus gracilis. Evidence for growth of fasciculus gracilis axons into the caudal medulla was also seen in cases lesioned at PD8. In contrast, labeled axons were not observed rostral to the lesion when it was made at PD12 or at later stages of development. In order to verify that some of the axons which crossed the lesion originated within dorsal root ganglia, the thoracic cord was transected at PD5 in another group of animals and 7 days later, injections of CT-HRP were made into one of the hindlimbs. After a 3 day survival, labeled axons could be traced through the lesion site and into the caudal medulla. We conclude that axons of the fasciculus gracilis reach the nucleus gracilis by at least PD5 in the opossum and that they grow through a lesion of their spinal pathway when it is made at the same age or shortly thereafter. The critical period for such growth appears to end between PD8 and PD12.