The ontogeny of ascending and descending spinal pathways was examined in bullfrog (Rana catesbeiana) tadpoles using the transported histochemical marker, horseradish peroxidase (HRP). The adult pattern of brainstem projections to lumbar spinal cord is evident as early as larval stage I (Taylor and Kollros, Anat. Rec., 94:7-24, 1946), although the number and size of projecting cells increases as the animal matures. These projections arise from presumptive hypothalamic neurons at the diencephalic-mesencephalic border as well as from neurons of the vestibular nucleus, oculomotor nucleus, and reticular formation. In contrast to the stability of the pattern of descending projections, the sources of fibers ascending to the brainstem change during larval life. In early larval stages, brainstem projections from lumbar spinal cord arise primarily from Rohon-Beard cells and neurons of the superficial dorsal horn. In later stages, neurons in the intermediate and ventral areas of the spinal gray can also be retrogradely labeled by HRP application to the brainstem at the level of the VIIIth nerve. Evidence of the existence of dorsal column and lateral cervical nuclei in adult frog and tadpoles older than stage VIII is presented. The ascending projections of embryonically born primary neurons were also investigated. Rohon-Beard cells, which are sensory neurons with their cell bodies in the spinal cord, were found to send ascending processes as least as far rostral as the level of the VIIIth nerve entry zone. Anterolateral and dorsal marginal cells, probable homologs, respectively, of mammalian spinal border cells and cells of Waldeyer (1888), were also found to project rostrally at least to the rhombencephalon. These marginal cells persisted through metamorphosis into adulthood.