A significant number of diploid oocytes is ovulated from adult NMRI/Han mice treated with high doses of gonadotrophins. This inhibition of the first meiotic division is very likely caused by an altered communication between the germ cell and the surrounding somatic cells leading to a failure of the endocrine control of meiosis. The present study examined the role of mitochondria in follicular development, oocyte maturation and chromosomal segregation during first meiotic division in NMRI/Han mice. To affect mitochondrial function during the late phase of follicular maturation, chloramphenicol, a potent inhibitor of mitochondrial peptidyl transferase, was used. Adult mice were treated with chloramphenicol (CAM; 18.8 or 37.5 mg/kg b.w.) at different times after the pregnant mare serum injection. The results revealed that CAM inhibited the characteristic increase of ovarian weight, reduced the number of oocytes ovulated per female, lowered the progesterone concentration in the postovulatory ovary and increased the incidence of ovulated diploid oocytes. It was concluded that an irregular mitochondrial function may affect normal follicular development and oocyte maturation, and potentially interferes with the order chromosome segregation during the first meiotic division.