No nitrite was detected in the tissues or contents of the gastro-intestinal tracts of normal rats but after 2 wk on a diet containing 0 . 5% sodium nitrate the levels of nitrite in the stomach, small intestine and large intestine contents were 0 . 83%, 1 . 64-2 . 07 and 0 . 83 micrograms/g of contents respectively. Concurrent administration of 2% Lactobacillus preparation and 0 . 5% sodium nitrate in the diet for 2 wk further increased the nitrite levels in the intestines and slightly increased the level in the stomach. The elevation of nitrite levels induced by sodium nitrate administration was potentiated considerably by combined treatment with sodium bicarbonate and hetacillin producing nitrite levels of 3 . 16, 2 . 93-5 . 18 and 1 . 96-2 . 34 micrograms/g of the contents of the stomach, small intestine and large intestine respectively. Like hetacillin, minomycin and thiamphenicol also potentiated the nitrite production whereas amikacin (another antibiotic) strongly inhibited the formation of nitrite in the stomach. The different effects of the antibiotics may be due to their selective activities on the various microbes. The results indicate that the levels of nitrite in the gastro-intestinal tract are regulated by the level of nitrate intake, the population of microflora and the gastric pH. The safety of combined medication with antacids, antibiotics and Lactobacillus preparations in man deserves further investigation.