Messenger RNA coding for BNP precursor has been detected and mapped in histological sections of rat brain by in situ hybridization using specific rat BNP DNA probes labelled with 32P. The results showed that areas of highest concentration of BNP gene expression were located in the hypothalamus and in the verge area of the thalamus. Successively decreased concentration could also be found in amydaloid nuclei, olfactory bulb, hippocampus and cerebral cortex. While practically no BNP gene expression could be found in cerebellum and medulla oblongata.