Embryonic development is a critical period that determines the quality of juvenile fish. Our objective was to determine the mechanisms modulating metabolism during embryonic developmentA number of key enzymes play an important role during the process of embryonic development, including MDA) content reflects the level of lipid peroxidation, which results from the reduction of antioxidant defenses. healthy embryos at several stages following fertilization and measured LSZ and LSZ and CAT activity decreased gradually during development of the embryos. LSZ activity was lowest after 96 h, i.e. (0.450±0.064) U/mg. Furthermore, LSZ activity was lower at 96, 120, and 144 h than that at 24 h (<0.05). CAT activity also decreased significantly after 96, 120, and 144 h (<0.05), relative to levels in 24 h old embryos. In contrast, MDA content increased gradually throughout embryonic development, with the largest increase occurring after 96 h. MDA content was significantly higher after 120 and 144 h than after 24 h (<0.05). In summary, LSZ and CAT activities gradually decreased during development and were generally significantly lower than initial levels by 96 h. This decrease is likely due to metabolism of maternal enzymes and the poor ability of the embryos to synthesize new enzymes. The decrease in CAT activity likely resulted in a surplus of reactive oxygen species, leading to the accumulation of MDA. Our results suggest that the embryos were subject to certain level of oxidative stress. Furthermore, it appears that the first 96 h of embryonic development is critical in determining the future of embryonic development. Therefore, goldfish embryos should be handled with care during this period to minimize stress. The duration of embryonic development should also be maintained within a suitable range by manipulating temperature because the extension of developmental time will result in oxidative stress and increase the risk of mortality in the embryos.