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Transplantation of fetal nigral dopamine neurons into the caudate and putamen of Parkinson's disease patients produces limited symptomatic relief. One approach to augment the outgrowth and function of nigral grafts includes exposure of the graphs to neurotrophic factors; however, the temporal requirements for optimizing these actions are unknown. The present study characterized the ontogeny of brain-derived neurotrophic factor (BDNF) in the rat striatum and used this information to define and evaluate three distinct periods of BDNF infusion into fetal nigral grafts transplanted into the striatum of rats with experimental Parkinson's disease. At postnatal day 1 (P1), BDNF and dopamine were measured at 17 and 27% of peak levels, respectively, that occurred at P27 for both. Both compounds showed their greatest surge between P7 and P20, increasing from 40% to approximately 95% of peak levels. Exogenous BDNF infused into transplants during weeks 1 and 2 after the transplantation, which coincide with the developmental period embryonic day 14 (E14)-P7 for transplanted tissue, did not improve rotational behavior or enhance fiber outgrowth of transplanted dopamine neurons. Delaying the BDNF infusion until transplanted tissue was approximately P8-P21 greatly enhanced the effect on rotational behavior and doubled the area of dopamine fiber outgrowth from the transplants. Delaying the infusion until transplanted tissue was approximately P36-P49 failed to augment fiber outgrowth and decreased the behavioral function of transplants. Thus, the optimal effect of exogenous BDNF on the development of dopamine neurons in fetal nigral transplants occurs at a postnatal age when endogenous dopamine and BDNF show the greatest increases during the normal development of the striatum.