P19 is a pluripotent stem cell line which can be induced to differentiate along either mesodermal or ectodermal lineages. When treated with retinoic acid, P19 cultures differentiate into cell types similar to those derived from neuroectoderm. Lineage-specific markers have shown neurons and astroglia to predominate in these cultures. By manipulating the growth conditions, differentiated cultures can be greatly enriched for neurons. In serum-free medium, few nonneuronal cells survive while the developing neurons can be seen to form extensive networks in the dish. The neurons obtained are irreversibly postmitotic and have many of the characteristics of mature neurons of the CNS. They not only express a number of neuron-specific gangliosides and proteins but also undergo an electrophysiological maturation of their membranes. Depolarizing stimuli elicit full-blown action potentials in mature P19-derived neurons and spontaneous post-synaptic potentials have been recorded. As undifferentiated P19 cells proliferate rapidly, they can be cultured with relative ease. Once exposed to retinoic acid, neurons develop within 4 to 5 days. Using standard transfection protocols, foreign DNA sequences can be introduced into P19 cells, thereby opening neuronal function and differentiation to genetic analysis.