Pre- and postnatal injections of nerve growth factor, initiated with one dose on day 17 of gestation and continued after birth with daily subcutaneous administration until day 10 of life, produce massive transformation of chromaffin in sympathetic nerve cells in the rat adrenal medulla. Large sympathetic ganglia, absent in controls, adhere to the medial external surface of the gland. Nerve fibers produced by the transformed chromaffin cells invade the inner and outer cortical zones of the organ, producing cell depletion and substantial alteration of the structure of the cortical layers. When the growth factor treatment is initiated after birth, only a partial replacement of chromaffin with nerve cells takes place. The treatment is ineffective after the second postnatal week. Injections of a specific antiserum to nerve growth factor in 17-day-old rat fetuses, which were continued after birth, produce progressive and massive destruction of chromaffin cell precursors and of immature chromaffin cells in the adrenal medullary gland.