The cardiac neural crest contains ectomesenchymal and neural anlagen that are necessary for normal heart development. It is not known whether other regions of the neural crest are capable of supporting normal heart development. In the experiments reported herein, quail donor embryos provided cardiac, trunk, or mesencephalic neural crest to replace or add to the chick host cardiac neural crest. Neither trunk nor mesencephalic neural crest was capable of generating ectomesenchyme competent to effect truncal septation. Addition of mesencephalic neural crest resulted in a high incidence of persistent truncus arteriosus, suggesting that ectomesenchyme derived from the mesencephalic region interferes with ectomesenchyme derived from the cardiac neural crest. Derivatives from the trunk neural crest, on the other hand, did not result in abnormal development of the truncal septum. While mesencephalic neural crest seeded the cardiac ganglia with both neurons and supporting cells, this capability was limited in the trunk neural crest to the more mature regions. These studies indicate a predetermination of the ectomesenchymal derivatives of the cranial neural crest and a possible competition of neural anlagen to form neurons and supporting cells in the cardiac ganglia.