Interpretation of stress responses in wildlife is inadequate due to the range of natural variation and potential confounds of individual and life-history variables. In marine mammals, endocrine response data are sparse and variable across species. Blood adrenal and thyroid hormones were measured in 144 chemically immobilized yearling elephant seals at Año Nuevo State Reserve to characterize variation between sexes and across semiannual haul-outs. There was no relationship between hormone concentration and time needed for collecting blood or diel pattern, suggesting that concentrations represented baseline values. Serum cortisol concentrations did not vary with gender or across fasts but increased dramatically during molting. Cortisol was significantly correlated with aldosterone at all measured life-history. Thyroxine levels were lower in females and decreased with fasting in both sexes during the Fall haul-out. Cortisol concentrations were correlated with reverse T3 concentrations across all measured life-history stages suggesting an important impact of cortisol on deiodinase enzymes and thyroid function. Significant variation in stress hormone concentrations with gender and life- history stage emphasizes the importance of contextual variables when interpreting serum hormone concentrations
