The studies described here were carried out in the Neuroregulation Group at the Department of Physiology of the University of Leiden. Over the last decade this group has, in close collaboration with the departments of neurosurgery and urology of the Academic Hospital of Leiden, studied development and regeneration of the spinal cord and its peripheral nerves both from a neuroanatomical and a clinical perspective. During this period the development of brainstem projections to the spinal cord of the rat was studied with a retrograde tracing technique. Horseradish peroxidase was injected into the spinal cord of rat fetuses, both at different ages, and at different levels of the spinal cord. These studies aimed to discover regularities in the behavior of descending fiber systems that could yield insight into the logic that the nervous system must employ to structure its connectional pattern during development. Such insight might then be applied to improve regeneration of the nervous system.