Developing neurons contain high levels of several proteins which are absent or relatively scarce in mature neurons. GAP-43 is a cytoplasmic protein primarily found within neurons; high levels of this protein are correlated with axonal elongation or regeneration. 5B4-CAM, a glycosylated transmembrane protein, is a member of the NCAM family present in growth cones and in plastic CNS structures. Antibodies directed against these two developmentally regulated proteins were used to characterize the time-course of maturation of transplanted fetal mesencephalic neurons. For our experiments unilateral injections of 6-hydroxydopamine were made into the nigrostriatal bundle in Sprague-Dawley rats. The effectiveness of the lesion was verified by apomorphine-induced rotation and by postmortem examination of the substantia nigra. Following behavioral testing, pieces of ventral mesencephalon obtained from E15 fetuses were transplanted into the caudoputamen ipsilateral to the lesion. Immunocytochemistry revealed high levels of GAP-43 and 5B4-CAM at 5, 11, and 15 days post-transplant but relatively lower levels by 3 weeks. At 13 weeks the immunoreactivity present within the transplant tissue was approximately equal to that found within the host striatal neuropil. This time-course of higher GAP-43 and 5B4-CAM immunoreactivities coincides with the time-course of neuritic outgrowth of dopamine containing cell populations within the ventral mesencephalon in situ as well as within ventral mesencephalic transplants. This implies that axon elongation occurs over a period similar to that which occurs during normal development. These data suggest that the effects of transplantation surgery and the altered environment of the host striatum do not significantly affect the time-course of development of ventral mesencephalic neurons.