Isotopic and colorimetric in situ hybridization techniques were combined to determine if nerve growth factor (NGF) mRNA is colocalized with mRNA for the GABA biosynthetic enzyme glutamic acid decarboxylase (GAD) in adult rat hippocampus. Quantification of neurons labeled with both 35S-labeled GAD67 mRNA and digoxigenin-labeled NGF cRNA determined that of the NGF cRNA-labeled neurons, 97% within regions CA3-CA1, and 88% within the hilus, were also labeled with GAD67 cRNA. Overall, 47% of the total population of GAD67 cRNA labeled cells were NGF cRNA positive. The greater portion of stratum granulosum was lightly labeled by the NGF cRNA alone. The results indicate that, excepting stratum granulosum, NGF is predominantly synthesized by GABAergic neurons in rat hippocampus.