Nitrate balance was measured in nine men consuming a fixed daily diet with constant nitrate (570 mumol/day), varying nitrite (18 to 150 mumol/day), and varying erythorbate levels. Nitrite and erythorbate were added to meat that was then cured and frozen until consumption. All diets were consumed by each subject for 17 days each. Average daily urinary nitrate excretion ranged from 959 to 2382 mumol/day. Subjects excreted significantly more nitrate in urine when fed nitrite cured meats with or without erythorbate than when fed uncured meat (1617 and 1577 versus 1430 mumol nitrate/day, respectively). The amount of nitrate excreted in urine consistently exceeded intakes of nitrate and nitrite by an average 870 mumol/day. This excess represented endogenous synthesis by subjects and was not due to unmeasured nitrate in the diet. The commonly used Greiss and xylenol procedures were unable to measure all nitrate in urine and in diets when compared to high performance liquid chromatographic analyses. The Greiss and xylenol analyses underestimated nitrate synthesis by 220 and 150 mumol/day, respectively when results were not adjusted by determining recovery of added nitrate.