Growth-associated protein-43 (GAP-43) is a phosphoprotein of the nerve terminal membrane which has been linked to the development and restructuring of axonal connections. Using a monospecific antibody prepared in sheep against purified GAP-43, we examined the temporal and spatial changes in the distribution of this protein from embryonic stage day 13 (E13) to adulthood. At stages in which neurons are still dividing and migrating, levels of GAP-43 are extremely low, as is seen in the cortical plate throughout the embryonic period. With the onset of process outgrowth, intense GAP-43 immunoreactivity appears along the length of axons: by E13, such staining is already strong in the brainstem, where it continues up through the first postnatal week and then disappears. In the neocortex, intense fiber staining first appears several days later but ends at the same time as in the brainstem. At the end of the period of intense axonal staining there is a brief interval in which high levels of GAP-43 immunostaining are seen in the neuropil. In regions of the brain in which specific developmental events have been characterized anatomically and physiologically, the period of dense neuropil staining coincides with the formation of axonal end-arbors, the beginning of synaptogenesis, and the time at which synaptic organization can be modified by the impingent pattern of activity (i.e. the critical period). Over the next few days, staining in neuropil declines sharply in most regions except for certain structures in the rostral neuraxis which may be sites of ongoing synaptic remodeling.