Expression of GAP-43, a neuronal specific growth associated phosphoprotein, has been highly correlated with the growth and remodeling of the nervous system during development and regeneration. As part of an effort to understand mechanisms of developmental plasticity in the somatosensory system, we determined how the expression of GAP-43 is affected by prenatal and early postnatal nerve cut and repair in macaque monkeys. We also observed normal developmental changes in the expression of GAP-43 during early postnatal life in macaque monkeys. The normal cuneate nucleus, as well as other nuclei of the ascending somatosensory pathways, had low levels of GAP-43 at birth that increased by 3 months and declined thereafter to reach adult levels between 8 and 15 months of age. Fiber tracts expressed low levels of GAP-43 at all postnatal ages, except the pyramidal tract which demonstrated high levels at birth that decreased over the first year. These observations suggest a gradual but differential synaptic maturation in lower brain stem nuclei as macaque monkeys mature. Greatly increased levels of GAP-43 were observed at the time of birth in the cuneate nucleus of two macaque monkeys with prenatal (E94 and E114) nerve repair. Such an increase was not found after prenatal nerve repair with a postnatal survival time of 15 months, or after early postnatal nerve repair with short (80 days) or long (20 months) survivals. The results suggest that reorganization mechanisms at central terminals of peripheral nerves are very different following prenatal than postnatal nerve damage. © 1995.