The principal pathway for the movement of water and cryoprotectants in oocytes/embryos at various stages helps in the selection of suitable cryoprotectant(s) and optimal conditions for cryopreservation. The permeability to water and cryoprotectants of mammalian oocytes/embryos can be measured as changes in volume in hypertonic solutions containing sucrose or cryoprotectant. A longer exposure to the cryoprotectant solution(s) would be necessary to dehydrate the cells and allow the cryoprotectant(s) to permeate sufficiently. For vitrification of oocytes/early embryos, a two-step treatment would be effective, in which oocytes/embryos are first pretreated in a solution containing a low concentration of cryoprotectant for permeation in less toxic conditions, and then in the vitrification solution for a short time to cause the embryos to shrink by rapid dehydration. Conditions suitable for the cryopreservation of mammalian oocytes and embryos differ among maturational/developmental stages even in the same species.