1. Groups of four conventional (CV) rats ate natural or purified diets either with or without 100 g fat/kg and drank 0.235 M-sodium nitrate. The fats tested were butterfat, coconut oil, olive oil, maize oil and safflower oil.
2. Decreased urinary excretion of N-nitrosoproline (NPRO) was observed in rats fed on fat-supplemented diets compared with those fed on low-fat diets, with butterfat having the greatest effect of the fats tested.
3. Reduced excretion of NPRO was not the result of inhibition of the intragastric N-nitrosation reaction or absorption of nitrosamine from the gastrointestinal tract.
4. The availability of nitrite in aqueous solution was decreased by the fat diets but the effect was similar in all the fats tested.
5. Nitrate reductase activity was present in the forestomach contents of CV rats at pH > 4 and was apparently inhibited by feeding a fat diet. No nitrate reductase activity was detected in stomach contents of germ-free rats.
6. Nitrate reductase activity in stomach and small intestinal tissue was not altered by feeding a fat diet.
7. It was concluded that nitrate reductase activity in stomach contents was of microbial origin and the decreased urinary excretion of NPRO on feeding the fat diets was mainly due to the inhibition of nitrate reductase activity in stomach contents.