Abstract: Stress exposure and stress response are likely to vary according to life history strategies and environmental contexts. Although the acute release of glucocorticoids in response to unpredictable environmental conditions may lead to adaptative behavioral and physiological responses, exposure to high glucocorticoid levels on the long-term usually leads to deleterious cascading effects on animal’s physiology, behavior and fitness. Yet, some species may be more adapted to cope with challenging events than others and might naturally possess protective mechanisms limiting the adverse consequences of prolonged exposure to high glucocorticoid levels. King penguin chicks being naturally exposed to harsh weather, prolonged fasting and constant predation pressure, we investigated the life-history, behavioral and physiological consequences of an experimental increase in corticosterone levels on the medium and long-term (after removing implants), predicting to find relatively few costs associated with such treatment. Although non-significant, CORT-treated chicks were slightly larger, fledged slightly earlier and had slightly higher survival chances than placebo chicks. CORT-treated chicks were significantly more aggressive and displayed lower corticosterone response to acute stress on the medium term, while their physical activity and whole-body energetics were not significantly affected. While mitochondrial density was not significantly affected by the CORT treatment, CORT-treated chicks had lower oxidative stress levels on the long-term than placebo ones, and no significant impact was observed on telomere shortening. Finally, the expression of several genes (i.e. CORT receptor, HSP90, mTOR, NRF2 and TERF2) was up regulated in treated chicks, which could potentially mediate the apparent ‘stress’ resistance we observed at the phenotypic level.