Male Ola:SD rats were fed purified diets containing 5 or 20% lactalbumin as the source of protein, and the daily urinary excretion of nitrate and nitrosoproline was measured. Animals fed the high-protein diet consistently excreted more nitrate and nitrosoproline than littermates fed the low-protein ration, despite a similar, negligible amount of nitrate in both diets. Furthermore, whereas nitrite administration enhanced nitrosoproline excretion in both diet groups, nitrate administration increased nitrosamine output in the low-protein animals but did not affect nitrosation by rats given the 20% lactalbumin ration. Animals fed the 5% lactalbumin diet produced a smaller volume of urine than did the 20% diet group but other measurements of renal function were comparable for both treatments. The results suggest differences in endogenous nitrosation between rats fed diets high or marginal in protein, possibly reflecting decreased nitrate synthesis in the low-protein group.