We examined the reliability of using nitrogen concentration of stomach contents from hispid cotton rats (Sigmodon hispidus) as an index of dietary nitrogen. Stomach contents of hispid cotton rats fed pelleted diets varying in nitrogen concentration were analyzed for stomach nitrogen. Regression analysis revealed a positive linear relationship between stomach and dietary nitrogen, but the relationship was not 1:1. Thus, inverse estimation of the regression equation can be used to adjust for a lack of a 1:1 ratio to obtain more reliable and accurate estimates of diet quality. Although we expected this relationship to be robust in its application to field studies, the pelleted diet model consistently underestimated dietary nitrogen during model evaluation experiments with natural forages. We conclude that the applicability of using nitrogen concentration of the stomach contents of cotton rats as an index to dietary nitrogen is dependent on the level of accuracy and precision required in estimating nitrogen concentration of foods consumed.