Accumulation of reactive oxygen species during aging leads to programmed cell death (PCD) in many cell types but has not been explored in mammalian fertilized eggs, in which mitochondria are "immature," in contrast to "mature" mitochondria in somatic cells. We characterized PCD in mouse zygotes induced by either intensive (1mM for 1.5h) or mild (200mM for 15min) hydrogen peroxide (H2O2) treatment. Shortly after intensive treatment, zygotes displayed PCD, typified by cell shrinkage, cytochrome c release from mitochondria, and caspase activation, then terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) staining in condensed pronuclei. On the other hand, after mild treatment, zygotes arrested developmentally and showed neither cytochrome c release nor caspase activation over 48h; until 72h, 46% zygotes exhibited TUNEL staining, and 88% of zygotes lost plasma membrane integrity. Interestingly, mild oxidative treatment induced a decline in mitochondrial membrane potential and disruption of the mitochondrial matrix. Taken together, these results suggest that oxidative stress caused by H2O2 induces PCD in mouse zygotes and that mitochondria are involved in the early phase of oxidative stress-induced PCD. Furthermore, mitochondrial malfunction also may contribute to cell cycle arrest, followed by cell death, triggered by mild oxidative stress.