The effects of an 8-hour infusion of somatostatin, saline, gastrin or a mixture of gastrin + somatostatin on DNA synthesis and mitotic activity of gastrointestinal progenitor cells were explored in conscious rats, using in vivo labeling with 3H-thymidine and radioautography. Infusion of somatostatin (day or night) was shown to transiently reduce nuclear uptake of 3H-thymidine and cell division in both fundic and antral progenitor cells. Cell DNA synthesis in the gastrin + somatostatin-stimulated stomach was lower than in the gastrin-stimulated stomach. In the intestine, nocturnal infusion of somatostatin decreased DNA synthesis whereas diurnal infusion decreased cell division but no effect was observed on gastrin stimulation. Evidently, somatostatin may inhibit, perhaps indirectly, the cell proliferation in digestive mucosae and antagonize the trophic activity of gastrin but only in fundic and antral mucosae.