Many types of glial-neuronal interactions occur during the development of the nervous system. To determine how such interactions might affect the development of autonomic ganglia, we compared the morphology of embryonic rat sympathetic neurons grown in the absence and in the presence of ganglionic nonneuronal cells in serum-free medium. Dye injections, electron microscopy, and immunocytochemistry were used to distinguish axons from dendrites. In cultures without nonneuronal cells, most (greater than 80%) sympathetic neurons extended only a single axonal process, and this unipolar state persisted for at least 8 weeks. Coculture with ganglionic nonneuronal cells caused sympathetic neurons to become multipolar and to extend multiple (range 1-17) dendrites. Morphometric measurements made after 1 month of coculture indicated that the amount of dendritic growth that occurred in vitro (mean number of dendrites/cell = 7.5; total dendritic length = 1,050 micron) was similar to that normally occurring during a comparable period in situ. In contrast to its prominent effects on dendritic growth, coculture did not cause changes in the number of axons/neuron or in the uptake of neurotransmitter. Cultures with ganglionic nonneuronal cells were immunostained for antigens present on the surfaces of fibroblasts (Thy-1.1, fibronectin) and of glia of the peripheral nervous system (laminin). Fewer than 1% of the nonneuronal cells displayed immunoreactivity for fibroblastic antigens; in contrast, greater than or equal to 99% reacted with antibody to laminin. Moreover, reconstitution experiments revealed that purified populations of laminin-positive Schwann cells promoted dendritic growth. Fibroblasts and heart cells lacked this activity. These data indicate that glia selectively promote dendritic development in sympathetic neurons maintained in serum-free medium.