The neural crest gives rise to three major adrenergic cell types: sympathetic principal neurons, adrenal chromaffin cells, and small intensely fluorescent (SIF) cells. All of these derivatives synthesize and store catecholamines, but they differ in numerous other characteristics. SIF cells appear intermediate in phenotype between the other two. We have examined the role of several environmental factors in the differentiation of sympathetic principal neurons and adrenal chromaffin cells. In previous studies of young rat adrenal chromaffin cells in dissociated cell culture, differentiated characteristics such as the presence of the enzyme phenylethanolamine N-methyltransferase (PNMT), epinephrine (E) synthesis, and large catecholamine storage vesicles were not well maintained. Here we describe long-term culture of chromaffin cells which, in the presence of micromolar glucocorticoid, maintained all of these characteristics. In addition, chromaffin cells of a variety of ages were found to be dependent on glucocorticoid for long-term survival in culture. In the absence of glucocorticoid, many adrenal chromaffin cells from neonatal rats could be rescued by nerve growth factor (NGF) administration. They extended neurites, as previously described by Unsicker and colleagues (Unsicker, K., B. Krisch, U. Otten, and H. Thoenen (1978) Proc. Natl. Acad. Sci. U.S.A. 75: 3498–3502). In contrast to previous studies, however, with long-term exposure to NGF these cells became indistinguishable from mature sympathetic neurons, as judged by the following morphological and biochemical criteria: increased cell size and loss of intense CA fluorescence in their cell bodies; acquisition of characteristic neuronal ultrastructure, including morphologically specialized synapses; loss of chromaffin granules, PNMT, and E synthesis; and acquisition of neuron markers, including tetanus toxin labeling and immunoreactivity to neurofilament protein. This conversion to neurons was markedly enhanced by addition of a non-neuronal cell conditioned medium (CM) containing a neurite-promoting factor, which acted by increasing the NGF responsiveness of the chromaffin cells. Even chromaffin cells from adult rats, which are known to grow few processes in response to NGF alone, became neuronal in the presence of this CM plus NGF. While converting to neurons, adrenal chromaffin cells transiently assumed an intermediate phenotype resembling type I SIF cells, which suggests particular developmental relationships between the different cell types of the sympathoadrenal lineage.