Abstract: Summary Individuals must trade?off between energetically costly activities to maximize their fitness. However, the underlying physiological mechanism remains elusive. Oxidative stress, the imbalance between reactive oxygen species production and antioxidant and/or repair activities, has been suggested to underlie life?history trade?offs: greater investment in reproduction supposedly generating higher oxidative damage, thus reducing life span. While most studies used natural or experimental variation in offspring number to examine how reproduction affects oxidative stress, none studied the impact of offspring sex, although it could influence physiological costs and fitness, if the sexes differ in terms of energetic cost. Here, we aim at further understanding how reproduction (in terms of offspring sex, experimentally manipulated and number, not manipulated) influences oxidative stress in a wild seabird, where sons are energetically costlier than daughters. We did so by conducting a chick fostering experiment (to disentangle foster and produced sex ratio) and using four oxidative stress markers plus baseline corticosterone. First, the results suggest that individual physiological state before laying modulates upcoming reproductive effort. Individuals with higher pre?laying baseline corticosterone and lower antioxidant activity, estimated by their superoxide dismutase activity, subsequently invested more in reproduction, estimated by the cumulative number of days spent rearing chicks. Hence, it seems that only individuals that could afford to invest heavily in reproduction did so. Then, we examined the effects of reproductive effort on individual physiological state at the end of the breeding season. Higher reproductive effort seemed to imply higher physiological costs. Oxidative stress, estimated by the ratio of oxidized over reduced glutathione, increased with more male?biased foster sex ratio among mothers but not among fathers, whereas baseline corticosterone did so in both sexes. Similarly, lipid oxidative damage to red blood cells increased with increasing cumulative number of days spent rearing chicks. Our study provides the first evidence that brood sex ratio variation can affect oxidative balance, potentially in a sex?specific way, although more studies are needed to understand whether the observed physiological costs could lead to fitness costs. It also highlights the need to consider sex ratio in future studies investigating the role of oxidative stress in life?history trade?offs. A lay summary is available for this article.