The tenuous linkage of toxicant-induced biochemical responses to ecologically relevant consequences has lead to criticism of biomarker use in ecological assessment. Ecological effects are inferred from biomarker data when correlations are found with survival or fecundity. This is based on the assumption that toxicant defense expends resources needed for reproduction and growth. Convincing evidence of alterations in use of resources, such as protein, is necessary to support linkage to organism health. In this work, shifts in the stable isotope ratios of nitrogen ( 15 N/14 N, or 15 N) were used to indicate alterations in protein metabolism after dietary mercury exposure. Under conditions of increased protein degradation, 15 N becomes elevated. Since mercury may increase protein degradation through binding to and damaging protein, exposure to elevated mercury is expected to increase 15 N in target organs and the acid-soluble metals-defensive peptides glutathione (GSH) and metallothionein (MT). Snowy egrets (Egretta thula) fed reference or high levels of dietary mercury showed elevated 15 N levels in livers of birds consuming the high-mercury diet. The 15 N of bulk liver and the acid soluble fraction of liver increased with liver mercury concentrations. The acid soluble fraction 15 N increased more dramatically than bulk liver 15 N. MT and GSH content in the high mercury diet group were not elevated. In conjunction with the elevated acid soluble fraction 15 N, the absence of differences in MT and GSH suggests that the relatively high turnover rate of these peptides may obscure early detection of metals-induced defense. Yearling largemouth bass (Micropterus salmoides) fed reference or high-Mercury diets had elevated GSH and bulk liver percent nitrogen in the high-Mercury group at 30 days. The 15 N enrichment of the acid soluble fraction was significantly elevated and correlated with muscle Mercury concentration and liver percent nitrogen. While MT did not differ between groups, MT concentrations were significantly correlated with both acid soluble 15 N and liver percent nitrogen. To determine if isotopically labeled diet is a useful marker for allocation of bulk dietary nitrogen, a subset of animals were gavaged with a feed pellet supplemented to 500 o /oo 15 N. Isotopic data from these 15 N dosed fish produced equivocal results.