The permeability of dechorionated one-cell and six-somite stage zebrafish (Brachydanio rerio) embryos to water and the cryoprotectant methanol at 22 o C was studied, using real-time video microscopy to determine the volumetric changes of the embryos during cryoprotectant exposure. The equilibrium volumetric behavior of the embryos and the Boyle-van't Hoff relationships were determined using sucrose as a nonpermeating compound. These showed the embryos to behave as nearly ideal osmometers over the range of 253-1724 mOsm, with osmotically inactive volumes of 72.9 and 82.6% for one-cell and six-somite stage embryos, respectively. The Boyle-van't Hoff relationship of the ovary eggs was also determined for comparison and their osmotically inactive volume was 63.9%. The Kedem-Katchalsky parameters of water permeability (L p ), cryoprotectant permeability (P s ), and reflection coefficient ( ) were determined using DIFFCHAM software. The parameters reported in this study are phenomenological parameters referring to the overall embryo response. The mean values of these parameters were L p = 0.34 and 0.35 (μm/min*atm), P s = 0.45 and 0.04 (μm/s), and = 0.88 and 0.93 for one-cell and six-somite stage embryos, respectively. While the water permeability of the dechorionated zebrafish embryos at different developmental stages remained relatively stable, the permeability to the cryoprotectant methanol (P s ) appeared to decrease during embryo development. The P s and values for methanol are the first reported for dechorionated fish embryos at these stages.