c-ret encodes a tyrosine kinase receptor that is necessary for normal development of the mammalian enteric nervous system. Germline mutations in c-ret lead to congenital megacolon in humans, while a loss-of-function allele (ret.k ⁻) causes intestinal aganglionosis in mice. Here we examine in detail the function of c-ret during neurogenesis, as well as the lineage relationships among cell populations in the enteric nervous system and the sympathetic nervous system that are dependent on c-ret function. We report that, while the intestine of newborn ret.k ⁻ mice is devoid of enteric ganglia, the esophagus and stomach are only partially affected; furthermore, the superior cervical ganglion is absent, while more posterior sympathetic ganglia and the adrenal medulla are unaffected. Analysis of mutant embryos shows that the superior cervical ganglion anlage is present at E10.5, but absent by E12.5, suggesting that c-ret is required for the survival or prolif-eration of sympathetic neuroblasts. In situ hybridization studies, as well as direct labelling of cells with DiI, indicate that a common pool of neural crest cells derived from the postotic hindbrain normally gives rise to most of the enteric nervous system and the superior cervical ganglion, and is uniquely dependent on c-ret function for normal develop-ment. We term this the sympathoenteric lineage. In contrast, a distinct sympathoadrenal lineage derived from trunk neural crest forms the more posterior sympathetic ganglia, and also contributes to the foregut enteric nervous system. Overall, our studies reveal previously unknown complexities of cell lineage and genetic control mechanisms in the developing mammalian peripheral nervous system.