Giant oocytes or two-cell embryos have been reported in various mammalian species. They may arise during multiplication of oogonia, after fusion of two oogonia or, more probably, when nuclear division is not accompanied by cytoplasmic division. The ultimate fate of these giant embryos is not well known. In our laboratory, giant two-cell mouse embryos have been occasionally observed. Recently, we observed two giant one-cell zygotes in the same species. Both showed two female pronuclei and one male pronucleus, as well as two second polar bodies localized at opposite poles of the embryo. These two giant zygotes showed normal viability and developmental capacity. Their triploid nature was confirmed by cytogenetic analysis. In order to study this interesting phenomenon in more detail, we produced giant oocytes containing two germinal vesicles by cell fusion and cultured them in vitro. About one third of them extruded two first polar bodies; in the second group only one polar body was observed, whilst the last group was without polar bodies. When parthenogenetically activated, the consistent answer analogical to that observed in "in vivo" oocytes was only observed when oocytes with two polar bodies were activated. The implication for IVF technologies is discussed.