The aim of this study was to examine whether protein products of a transgene are localized within axon terminals in transgenic mice. We have previously created transgenic mice containing a chimeric gene composed of the human dopamine beta-hydroxylase gene promoter and the human phenylethanolamine N-methyltransferase (PNMT) cDNA. In the present study, we used an antiserum that detects specifically human PNMT but not mouse PNMT, and examined immunocytochemically the hippocampal formation of the transgenic mice. At a light microscopic level, immunoreactivity of human PNMT was found in fiber plexuses in the outer molecular layer of the dentate gyrus, and in cell bodies of layer 2 of the entorhinal cortex. At an electron microscopic level, in the outer molecular layer of the dentate gyrus, human PNMT immunoreactivity was observed in axon terminals that formed synapses with dendritic spines. The present study provides the evidence for localization of the transgene's protein products in axon terminals, suggesting axonal transport of the products.