Gastrointestinal peptides and hormones are known to penetrate through the utero-placental barrier and regulate fetal development. In the present study, we tested permeation of maternal brain-derived neurotrophic factor (BDNF) to fetuses, using BDNF-gene deficient mice and exogenous BDNF administration. At embryonic day 13.5 (E13.5)-14.5, BDNF protein concentrations in the fetal brain of BDNF homozygous null mutant (bdnf (-/-)) were comparable to the levels seen in wild-type fetuses. After E17.5, BDNF protein levels in bdnf (-/-) fetal brain were still detectable but its levels were significantly decreased below those in wild-type brain. When recombinant BDNF protein was injected into pregnant wild-type mice carrying E14.5 embryos, BDNF protein levels in fetal brain were elevated dose-dependently. These findings suggest that maternal BDNF reaches the fetal brain through utero-placental barrier and might contribute to its development.