Neurons were dissociated from the sympathetic ganglia of embryonic chicks, and cultured in the absence of non-neuronal cells. Both nerve growth factor (NGF) and high concentrations of extracellular K+ supported neuronal survival, and these effects were independent of the presence of serum in the culture medium. Only 60% of the neurons survived in response to 35 mM K+, and survival was not increased when both NGF and K+ were present together. It was, however, possible to maintain essentially all the neurons in culture with either NGF or high K+ concentrations if the culture substrate had been pretreated with heart cell-conditioned medium (which did not itself support neuronal survival). These observations are consistent with a common mechanism of action of both K+ and NGF for the survival of cultured embryonic neurons.