The energy expenditure associated with breeding may have detrimental effects on the breeder's physiology. Some studies have focused on the relationship between breeding performance and health status, but information on the link with biochemical variables reflecting susceptibility to oxidative stress is scarce. Over two years (2005 and 2006), using several morphological, biochemical and hematological variables, we measured the association between several measures of breeding (laying date, clutch size, mean egg weight, brood size, number of fledglings, and mean weight before fledging) and health in the Great Tit (Parus major). The effect of raising a second brood on the breeders' physiological condition was also studied. Females' body-condition index tended to be positively correlated with breeding performance, whereas in males the correlation was negative. Females laying later had lower hematocrits and higher glutathione peroxidase activity (GSH-Px), and those raising larger broods had also higher GSH-Px activity and tended to have lower plasma protein. The effect of raising a second brood was reflected mostly in body reserves but varied by sex. Our study suggests that trade-offs between breeding activity and physiological condition of the sexes differ and that hematocrit and GSH-Px are sensitive indicators of the physiological condition of breeding females.