In situ hybridization histochemistry with labeled cRNA probes complementary to monkey brain derived neurotrophic factor (BDNF) mRNAs has been used to study the cellular localization and expression of this neurotrophin in the prefrontal cerebral cortex of fetal and adult monkeys. Expression could not be detected in prefrontal cortex before the 121st fetal day. Thereafter, in fetal life and in adulthood BDNF mRNA could be detected primarily in large, putative pyramidal cells of layers III and VI throughout the prefrontal cortex. The temporal course and cellular localization of BDNF expression suggests its association with the development and stabilization of specific connections in regions of cortex that display marked functional plasticity.