The kinetics of salivary and biliary excretion of inorganic nitrates and nitrites was studied in mongrel dogs after iv injection of these compounds labelled with 15N. There was strong salivary excretion of 15NO-3 after injection of Na15NO3 but little or no nitrites were excreted in the saliva, indicating that the nitrites present in the mouth are mainly the result of in situ transformation of nitrates into nitrites. In the bile, large quantities of nitrates were excreted when the dogs received 15NO-2, indicating endogenous oxidation of the nitrite.