cDNA probes encoding preproenkephalin A (PPA) were used to detect the mRNA codings within the sections of adrenal glands of guinea pigs, processing according to the following procedures; perfused the animals with Zambonis' or 4% paraformaldehyde fixative; rinsed the block of adrenal glands for 12 hours in O.1mol/L phosphate sodium solution; the block were cut at -15℃ in a cryostat; Then the sections were prehybridized and hybridized with the biotinylated cDNA encoding PPA. The sections were incubated in the solution containing streptavi- din-biotinylated HRP and chromostained with DAB. The positive reactive produ- cts appeared in the cytoplasm of about 90% of adrenal medullary cells.