The inability to grow neurons in culture in the absence of complex and undefined biological fluids (e.g. serum) has proved a major obstacle to a rigorous formulation of the growth, nutritional and hormonal requirements of the developing nervous system. We have been successful in maintaining dissociated chick dorsal root ganglia neurons in a serum-free, defined medium composed of F12 synthetic medium and, substituting for serum, a combination of hormones (insulin, PTH, triiodothyronine, TRH, somatomedin, hydrocortisone, testosterone) and other factors (transferrin). Not only were these hormones found to be sufficient for the maintenance of neurons in vitro, but, by the selective elimination of one component from the mixture, the role of specific hormones in neutral development could be grossly assessed. The elimination of insulin proved to be most inimical to neuronal survival.