Studies of postnatal chromaffin cells, sympathetic neurons and Small Intensely Fluorescent (SIF) cells have suggested that these cells develop from a common progenitor, the sympathoadrenal (SA) progenitor, whose fate is determined by the relative levels of nerve growth factor (NGF) and glucocorticoid (GC) in its environment (Unsicker et al., 1978, Proc. Natl. Acad. Sci. USA 75:3498-3502; Doupe et al., 1985a, J. Neurosci. 5:2119-2142). Recent studies have identified such a bipotential SA progenitor in the rat embryo. Surprisingly, this progenitor is initially unresponsive to NGF; neuronal differentiation is instead promoted by fibroblast growth factor (FGF). However, FGF appears to promote NGF responsiveness, suggesting that neuronal differentiation involves a relay or cascade of growth factor action. Furthermore, chromaffin cell differentiation appears to involve two sequential, GC-dependent events: the inhibition of neuronal differentiation and the induction of epinephrine synthesis. The former event is a prerequisite to the latter. Thus both the chromaffin and neuronal pathways of differentiation follow a series of dependent events, involving changes in the responsiveness of SA progenitors to environmental factors. Such changes correlate with changes in antigenic marker expression that can be observed in vivo. In addition to choosing between neuronal and endocrine fates, SA progenitors must also express an appropriate neurotransmitter phenotype. For example, sympathetic neurons can become either noradrenergic or cholinergic. This cholinergic potential is already present in uncommitted SA progenitors, as evidenced by their ability to synthesize acetylcholine. Recent studies suggest that these cells may have yet other developmental capacities, including the ability to synthesize serotonin. This capacity is consistent with the hypothesis that SA progenitors are closely related to progenitors of enteric neurons, an idea supported by recent observations using novel antigenic markers. The SA progenitor may be, therefore, a "master" neuroendocrine progenitor for the peripheral nervous system.