The distribution of the growth associated protein GAP-43 has been described in the brain and spinal cord of rats and other placental mammals but not marsupials. In order to provide such information, we employed a monoclonal antibody to immunostain for GAP-43 in the central nervous system of adult and developing opossums, Didelphis virginiana. The GAP-43 immunoreactivity was widely distributed in the brain of adult opossums, but it was particularly dense within specific layers of the olfactory bulb and hippocampal formation, layer I of the cerebral cortex, the bed nucleus of the stria terminalis, the nucleus accumbens, the striatum, the amygdala, the septum, the olfactory tubercle, medial parts of the preoptic area and diencephalon, the substantia nigra, the ventral tegmental area, the periaqueductal grey matter, the interpeduncular nucleus, the periventricular grey, the molecular layer of the cerebellum, the superior central nucleus, the basilar pons, the dorsal vagal and solitary nuclei, and laminae I and II of the spinal trigeminal nucleus. Immunoreactivity for GAP-43 was also present within the spinal cord, where it was densest within laminae I, II, IX, and X and within the intermediolateral cell column. In most areas of the brain and some areas of the spinal cord, an inverse correlation existed between the location of GAP-43 and myelin. Immunostaining for GAP-43 was found throughout most of the central nervous system during early development, but it decreased with age in a regionally specific manner until the adult pattern was reached. Our results suggest that the distribution of GAP-43 in opossums is similar in many respects to that reported in rats and that it is developmentally regulated.