Glutathione peroxidase (GSHPx) activity was investigated in the liver, red blood cells (RBC) and blood plasma (BP) of different chicken genotypes. Significant differences among genotypes and age groups were found in liver GSHPx activity during embryonic development and in day-old chickens. Breed, sex and age effects were found in RBC and BP from one day of age until peak egg production. A negative correlation was found between embryo liver GSHPx activity and egg weight (-0.24; P≤0.01), between RBC and BP GSHPx activity and body weight (-0.32 and -0.44; P≤0.01), and between liver and RBC GSHPx activity (-0.54; P≤0.01), while positive correlations were demonstrated between liver and BP (0.66; P≤0.01) and BP and RBC (0.32; P≤0.01) GSHPx activity. In conclusion, variation in GSHPx activities in different chicken breeds during development suggests that it is genetically regulated. This finding indicates that GSHPx activity may be useful in selection.