Nuclear and pronuclear transfer procedures were used to assess the functional competence of the nucleus and cytoplasm of mouse germinal vesicle-stage oocytes denuded of granulosa cells and matured in vitro or in vivo before artificial activation using a sequential treatment of A23187 cycloheximide. Following activation, in vitro-matured oocytes were “fertilized” by inserting a male pronucleus (PN), cultured to the 2-cell stage, and then transferred to the oviducts of foster mothers. No live births were noted, whereas a 17% live birth rate was observed when in vivo-matured oocytes were used. The developmental competency of other zygotes was similarly assessed following the exchange of haploid PN of matured and activated eggs with the female PN of fertilized zygotes. When PN of oocytes subjected to maturation and activation in vitro were transferred, only 1 of 79 reconstructed zygotes developed to term. In contrast, the live birth rate was 21% (11 of 53) for zygotes reconstructed with PN from in vivo-matured oocytes. Moreover, a live birth rate of 23% (8 of 35) was observed for reconstructed zygotes with female PN from “hybrid” oocytes created by transferring the metaphase II nuclei of in vitro-matured oocytes into enucleated, in vivo-matured oocytes before activation. Such results suggest that the nucleus of an in vitro-matured oocyte can support embryonic development, but only when it is activated in the proper ooplasmic milieu. The cellular factors creating this ooplasmic milieu appear to develop normally in vivo during follicle maturation to metaphase II, but they fail to do so when the oocytes are denuded of granulosa cells and cultured in vitro before the final stages of maturation. In parallel studies, male and female PN of in vivo-fertilized zygotes were inserted into oocytes that were activated and enucleated following either in vitro or in vivo maturation. Live birth rates were comparable at 19% (5 of 27) and 18% (9 of 49), respectively, suggesting that, regardless of the environment of the final stages of oocyte maturation, the resultant ooplasm is competent to support all aspects of embryonic development once activation and PN formation has been completed. Such findings only point further toward the importance of the condition of the ooplasmic milieu at the time of chemical activation. Whether a similar situation exists when eggs are activated following sperm penetration remains to be determined.