The development of superior cervical ganglion cells has been studied in the fetal rat. Sympathetic cells appear first in thoracic sites and, one day later, in cervical sites; localized proliferation among these cells gives rise to the superior cervical and stellate ganglia. The maturation of superior cervical ganglion cells was examined by staining these neurons with horseradish peroxidase in fetal preparations maintained in vitro. This method showed that cells begin to extend processes at widely different times, without regard to a given cell's position in the ganglion. Postganglionic axons appear as early as day 12 of gestation (E12), when only a small number of ganglion cells have emerged from the mitotic cycle. The axon generally originates from a point on the ganglion cell body that is oriented toward the route of subsequent axon extension. As the postganglionic axons grow, they do not branch within the superior cervical ganglion and branch only to a slight extent, if at all, within developing peripheral nerves. Axonal growth is rapid, and fibers reach relatively remote sympathetic targets as early as E15. Dendrites first appear on E14 and are elaborated by ganglion cells that have already extended their axons. By the end of gestation, the number of primary dendrites found on some cells already falls within the range found in maturity.