A procedure of in situ hybridization, termed hybridization histochemistry has been developed to locate in specially prepared whole sections of tissue those areas which contain specific mRNA populations, the presence of which would indicate that a certain gene is “switched on” and production of a specific protein or peptide is highly probable. In the main ³²P-labelled recombinant DNA probes were used. These probes were complementary to several peptide hormones or enzymes.³²P-labelled DNA probes were favoured initially because the short time needed for exposure enabled the development of the procedure to proceed quickly. The results indicate that the mRNA's of the species being examined are present in whole sections of tissue in a physical state which leaves them accessible to cDNA probes. The results are highly reproducible and specific. Because the method allows a clear distinction to be made between those cells which manufacture a peptide or protein and those which have imported the material it will have a special use in resolving many controversies over specific sites of origin of peptides and proteins, especially in tissues e.g. CNS and gut which are functionally complex and morphologically heterogeneous.