Effect of gastric mucus on the uptake of the carcinogen MNNG by gastric mucosal DNA. In prostaglandin E2 (PGE2)-, pirenzepine-, and indomethacin-administered rats, the incorporation of N-[methyl-3H]-N'-nitro-N-nitrosoguanidine ([methyl-3H]MNNG) into gastric mucosal DNA was measured quantitatively by liquid scintillation counting after intragastric instillation of [methyl-3H]MNNG. The amount of incorporation was 25.4 +/- 5.9 pmol/mg DNA in control rats, 11.7 +/- 3.8 pmol/mg DNA in PGE2-administered rats, 6.2 +/- 5.6 pmol/mg DNA in pirenzepine-administered rats, and 42.9 +/- 14.4 pmol/mg DNA in indomethacin-administered rats. PGE2 and pirenzepine significantly decreased the incorporation as compared with the control group. In contrast, indomethacin increased the incorporation. In addition, gastric mucosa of these drug-treated rats was studied histochemically. PGE2 and pirenzepine increased secretion of gastric mucus whereas indomethacin decreased it. It is possible that gastric mucus has a protective effect not only against ulcerogenic agents but also against carcinogens. It is considered that gastric mucus plays an important role in the defense mechanism against carcinogenesis. 