Non-radioactive in situ hybridization employing detection of biotin-labeled probes by an alkaline phosphatase-based procedure has proven useful for demonstrating a wide variety of mRNA species. With certain developers, the alkaline phosphatase reaction product is both light microscopically visible and fluorescent. We have exploited this to perform simple double-stainings for mRNA's and their corresponding peptide products in human insulin and gastrin cells and in rat ACTH, MSH and gastrin cells. Such stainings show that nearly all of these cells simultaneously contain both the peptide hormone and its corresponding mRNA. Human gastrin cells show a differentiated localization of gastrin mRNA and gastrin. Thus, while gastrin immunofluorescence predominates in secretory granules present in the basolateral region of the cells, gastrin mRNA is virtually restricted to the supranuclear region of the cells. Here it may be preferentially associated with granular endoplasmic reticulum. The strict subcellular localization of gastrin mRNA differs from that of general polyadenylated RNA in the G cells and raises questions whether specific transport routes or sites of accumulation for defined mRNA species exist.