We have demonstrated that ammonia is oxidized to nitrate in the rat. Male Sprague-Dawley rats gavaged with 1000 μmol N-15ammonium chloride each day for 5 days were found to excrete low, but significant amounts of excess N-15-nitrate in their urines on the five days of treatment and on the five subsequent days. We recovered a total of 0.28 ± 0.03 μmol excess N-15-nitrate (mean ± SE) per rat, which indicates that ammonia is converted to nitrate in a yield of ˜0.0080%. The oxidation of N-15-labeled glycine and L-glutamic acid to N-15-nitrate could not be detected. N-15-Hydroxylamine was oxidized in the rat to N-15-nitrate in a yield of 4.7%, which shows that hydroxylamine is a possible intermediate in tbe ammonia oxidation process. Injection of rats with Arochlor 1254, an Inducer of several isozymes of cytochrome P-450, did not significantly affect the rate of endogenous nitrate synthesis. Carbon tetrachloride, which causes hepatic lipid peroxidation, produced a small but significaot increase in nitrate synthesk. We confirmed the observation that a bacterial endotoxin can greatly stimulate nitrate synthesis, and we showed that concument treabnent with superoxide dhsmutase does not modify the effect of the endotoxin. An in vitro chemical model system was used to demonstrate that oxidation of ammonia to nitrate by the hydmxyl radical at physiological pH is chemically feasible. Our results are consistent with the hypothesk that ammonia is oxidhed to nitrate in vivo by a ooneqmatic process which invohes active oxygen species such as the hydroxyl radical. We estimate that a 215 g rat produces 3.0 μmol of nitrate per day via ammonia oxidation.