For better understanding of the role of platelet-derived growth factor (PDGF) B-chain in the brain, the expression of PDGF B-chain was studied in the mature rat brain at both protein and mRNA levels, by assay of PDGF B-chain-related mitogenic activity, Northern blot, in situ hybridization and immunohistochemistry. It was shown that (1) mature rat brain contained substantial PDGF B-chain-related mitogenic activity, (2) significant amounts of two sizes of transcripts (3.5 kb, widely, and 2.6 kb, weakly and in narrower areas) were expressed in the brain, and (3) the transcripts were localized in ubiquitous neurons by in situ hybridization, with the strongest signal in hippocampal pyramidal cells, which distribution almost corresponded with that of the immunoreactive products. The abundant neuronal localization of the transcript and protein, as well as the neuronal expression of the receptor reported elsewhere, suggests the role of the growth factor in neuronal cells as a neuronal regulatory and/or trophic agent acting by autocrine loop or by neuron to neuron interaction. However, there was an apparent discrepancy in part, in the distribution between transcripts and immunoreactivity; that is, transcripts were expressed intensely in the intermediate pituitary lobe with only a scattered immunoreactivity, and the opposite situation was observed in the accessory olfactory nerve and posterior pituitary lobe. This might suggest that PDGF B-chain is transported or secreted in these foci.