There is much concern about the role of nitrate in the formation of carcinogenic N-nitroso compounds. There has been renewed interest in the endogenous formation of nitrate arising as a host response to infection. This study was designed to investigate whether the large increases in nitrate excretion rate reported (6-15-fold) for certain infectious diseases is also a feature of systemic influenza infections. Volunteers were challenged either with an attenuated strain of influenza A virus or with saline; and excreted nitrate was measured in subsequent 24-h urine samples. Both with and without adjustment for potential confounding by dietary and other factors, it was clear that neither mild nor moderate influenza A virus infection resulted in substantial endogenous nitrate biosynthesis since all the variation in urinary nitrate excretion observed was within the range of normal daily fluctuations. It remains possible that a stronger and more consistent nitrate excretion response might be observed in other infectious illnesses with greater systemic disturbance.