The hormone gastrin is mainly produced by the G cells of the antral mucosa and plays a major role in the regulation of digestive mucosal growth. Since it permits identification of cell types containing mRNA, in situ hybridization (ISH) appears to be an interesting method for studying gastrin-producing tissues. In this study, in situ detection of gastrin mRNA has been carried out on frozen sections of four human normal antral mucosa samples and of six colonic carcinomas removed from patients with high levels of plasma gastrin, using a gastrin oligonucleotidic DNA probe. We have compared the results provided respectively by the [35s] labelling and the digoxigenin labelling of the synthetic probe. Positive cells were found in each normal sample analysed with radioactive- as well as digoxigenin-labelled antisense probes. The total number of cells expressing gastrin mRNA appeared slightly higher with the [35s]-labelled probe, while the digoxigenin-labelled probe gave a better definition of positive signals. In contrast, neither radioactive nor cold probes gave positive signals in the six colonic carcinoma samples, although gastrin expression had been demonstrated in these tumours using a reverse transcriptase-PCR method. These results show that, although ISH does not seem sensitive enough to allow the detection of very low levels of gastrin expression, it would appear to be a reliable method for visualizing gastrin mRNA in human antral mucosa.