Unlike most pathways of the mature mammalian central nervous system (CNS), the CNS of lower vertebrates can regenerate after jury, a capacity that may be due to the secretion of neurite-promoting factors from the injured CNS. We report that conditioned media (CM) from the injured optic nerve of the mature goldfish promoted marked neurite outgrowth from dissociated embryonic rat cortical and hindbrain neurons in serum-free, neuron-enriched culture. This property was not shared by CM from intact goldfish optic nerve, or from intact or injured optic nerve of mature rats. Neurite-promoting activity was obtained at concentrations as low as 100 ng total protein/ml of CM from injured goldfish optic nerve, and was associated with a distinctive morphology of neurite outgrowth. Due its properties of non-dialyzability, heat lability, and trypsin sensitivity, the neurite-promoting factor(s) appeared to be one or more protein species of MW greater than 12,000. Factors secreted by the regenerating CNS of lower vertebrates can directly promote outgrowth of mammalian CNS neurons.