In vivo reducing capacity and cytosolic glutathione peroxidase (GSH-Px) activity in liver homogenate were evaluated in 6 weeks old Se-deficient and normal rats. GSH-Px was significantly lower in Se-deficient rats than in normal rats. In vivo reducing capacity in head and liver parts, estimated from in vivo signal decay of a nitroxyl spin probe using a low frequency (300 MHz) ESR spectrometer, was significantly decreased in Se-deficient rats, suggesting a decrease of antioxidant capacity in Se-deficient rats.