Nerve regeneration and developmental outgrowth of axons are both correlated with increased synthesis of an axonal membrane protein designated GAP-43. Phosphorylation of an apparently identical protein, present at lower abundance in adult brains, has been correlated with long-term potentiation, a form of synaptic plasticity. We have now isolated a cDNA clone encoding GAP-43 from neonatal rat brain. The amino acid sequence is extremely hydrophilic, with no potential membrane-spanning domains and no sites for N-linked glycosylation, but with a short hydrophobic segment at the protein's amino terminus, consistent with a model in which GAP-43 extends from the cytoplasmic surface of growth cone and synaptic plasma membranes. Among several tissues and cells examined, GAP-43 mRNA is expressed only in neurons. Developmental and regeneration-associated changes in GAP-43 synthesis appear to be mediated largely at the level of transcription of a single gene.