The ability of neurons in the central nervous system to grow through a lesion and restore conduction has been analyzed in a developing spinal cord. The preparation consists of the entire central nervous system of the newly born opossum (Monodelphis domestica), isolated and maintained in culture. Cell division, cell migration, and reflexes are maintained in such preparations for up to 8 days in culture. In the present experiments, massive lesions were produced by crushing the spinal cord, which abolished all conduction for a day. By 2-3 days after injury, electrical conduction across the crush could be observed. After 4-5 days, clear recovery had occurred: the amplitude of the conducted volley was comparable to that in acute preparations. In such preparations, the spinal cord had largely regained its normal appearance at the crush site. Axons stained by carbocyanine dyes or horseradish peroxidase had, by 4 days, grown in profusion through the lesion and several millimeters beyond it. These experiments demonstrate that neurons in the central nervous system of newly born mammals, unlike those in adults, can respond to injury by rapid and extensive outgrowth in the absence of peripheral nerve bridges or antibodies that neutralize inhibitory factors of myelin. With rapid and reliable regeneration occurring in vitro, it becomes practicable to assay the effects of molecules that promote or inhibit the restoration of functional connections.