Outgrowth of distinct axonal and dendritic processes is essential for the development of the functional polarity of nerve cells. In cultures of neurons from the hippocampus, where the differential outgrowth of axons and dendrites is readily discernible, we have sought molecules that might underlie the distinct modes of elongation of these two types of processes. One particularly interesting protein is GAP-43 (also termed B-50, F1 or P-57), a neuron-specific, membrane-associated phosphoprotein whose expression is dramatically elevated during neuronal development and regeneration. GAP-43 is among the most abundant proteins in neuronal growth cones, the motile structures that form the tips of advancing neurites, but its function in neuronal growth remains unknown. Using immunofluorescence staining, we show that GAP-43 is present in axons and concentrated in axonal growth cones of hippocampal neurons in culture. Surprisingly, we could not detect GAP-43 in growing dendrites and dendritic growth cones. These results show that GAP-43 is compartmentalized in developing nerve cells and provide the first direct evidence of important molecular differences between axonal and dendritic growth cones. The sorting and selective transport of GAP-43 may give axons and axonal growth cones certain of their distinctive properties, such as the ability to grow rapidly over long distances or the manner in which they recognize and respond to cues in their environment.