Reactive oxygen species (ROS) are involved in the etiology of numerous diseases and are suggested to be one of the mechanisms of action of several teratogens such as cocaine, high concentrations of glucose and ketone bodies. We studied the antioxidant capacity of 9.5-12.5 day old rat embryos and their yolk sacs both in vivo and in vitro. We measured the activity of superoxide dismutase (SOD) and the hydrogen peroxide removing activity (mainly due to catalase (CAT) and glutathione peroxidase (GSH -Px) and found significant activity of these enzymes already at day 9.5 in the embryos and their yolk sac, both in vivo and in vitro. A gradual increase in the activity was found with the advancement of embryonic age. The reducing power, that reflects the concentration of low molecular weight antioxidants (LMWA) was measured by cyclic voltammetry. LMWA were found in the embryos and their yolk sacs on days 9.5-11.5 of gestation with the peak potential of 0.56- 0.62 Volts. On day 12.5 an additional group of LMWA appeared at a peak potential of 0.95-0.97 Volts. There was a gradual increase in the concentration of LMWA with the increase in embryonic age. Generally, the concentration of LMWA was higher in the embryo than in its yolk sac but it was similar in vivo and in vitro at the same developmental stage. The gradual development of the embryonic antioxidant capacity implies that under normal conditions the developing embryo is capable of coping with oxidative stress, but this may fail under various pathological conditions, leading to embryonic damage.