The course and termination of afferents in the spinal dorsolateral fascicle to some medullary sensory nuclei were studied by tracing degeneration following lesions of spinal white matter. The main conclusions depend on successive degeneration experiments; other points were studied with single-stage lesions. The dorsal column nuclei were particularly studied; terminations in these nuclei following dorsolateral lesions followed a clear-cut pattern, with fibres arising from segments below T6 terminating in the gracile nucleus and those with more rostral origin solely in the cuneate nucleus. In both nuclei, the major terminations were in their rostral third with most fibres traversing deep caudal regions where some termination also occurred. Some fibres ended contralaterally. These restricted regions of termination contrasted with the wide-spread terminations seen after lesions of the dorsal column. A region at the cuneate rostral pole, adjacent to but clearly separable from nucleus z, receives a dense projection from both caudal and rostral spinal levels, the former fibres terminating in the dorsal part of the region, the latter extending more ventrally. We treat this as a separate subnucleus. The afferents to the dorsal column (together with those terminating in the other nuclei studied) were confined to the extreme dorsolateral white matter. Our observations confirm the established view that only afferents arising from caudal segments (below at least T 4-5) terminate in nucleus z, and that afferents terminating in group x arise from all levels (at least between C5 and L5): also that neither receives any afferents through the dorsal columns. Dorsolateral fibres arising from segments above at least T6 terminate in a clear-cut area at the lateral border of the external cuneate nucleus. Heavy terminal degeneration was also seen in the lateral cervical nucleus of afferents arising from both above and below T 4-5.