ADULT rats with acute partial lesions of their upper thoracic spinal cords were implanted bilaterally with cell suspensions of 6–7 week-old embryonic human spinal cord tissue one segment above or below the lesions. After 14–19 weeks, the animals were perfusion-fixed and the tissue analysed with a light microscope after immunocytochemical labelling with an antiserum recognizing human, but not rat, intermediary neurofilaments. Using this method, extensive efferent projections were demonstrated extending longitudinally from the grafts into the host spinal cord, both in the caudal and rostral directions. Within the white matter tracts, dense bundles of fibres extended for about 3–4 mm, and single fibres were identified up to 10 mm away from the implants. Axonal growth of this length within host white matter has not previously been observed from intraspinal grafts of rat CNS tissue.