Changes were investigated in plasma metabolites and physiological and toxicological variables in rats fed for 2 wk on a basal diet or diets with 1.5, 5, 10, 15, and 30% added leucine. In the same experiment, the changes in gene expression in livers of rats fed the basal diet or diets with 5% and 15% added leucine were investigated using DNA microarrays. Cluster analysis of multivariate correlations of metabolites and physiological and toxicological variables indicated that the variables associated with excess nitrogen clustered together with leucine and alpha-ketoisocaproate. The gene expression data, although preliminary, indicated that there was little change in the expression of enzymes of the catabolic pathways for leucine but that there were changes in enzymes associated with nitrogen metabolism and other pathways downstream of leucine catabolism. The data seem consistent with excess leucine exerting its effects through the overloading of nitrogen metabolism and that urea or alpha-ketoisocaproate could be an early marker for the upper limit of adequate intake.