-Arsenic is a prominent environmental toxicant and carcinogen; however, its molecular mechanism of toxicity and carcinogenicity remains poorly understood. In this study, we performed microarray-based expression profiling on liver of zebrafish exposed to 15 parts/million (ppm) arsenic [As(V)] for 8 -96 h to identify global transcriptional changes and biological networks involved in arsenic-induced adaptive responses in vivo. We found that there was an increase of transcriptional activity associated with metabolism, especially for biosyntheses, membrane transporter activities, cytoplasm, and endoplasmic reticulum in the 96 h of arsenic treatment, while transcriptional programs for proteins in catabolism, energy derivation, and stress response remained active throughout the arsenic treatment. Many differentially expressed genes encoding proteins involved in heat shock proteins, DNA damage/repair, antioxidant activity, hypoxia induction, iron homeostasis, arsenic metabolism, and ubiquitin-dependent protein degradation were identified, suggesting strongly that DNA and protein damage as a result of arsenic metabolism and oxidative stress caused major cellular injury. These findings were comparable with those reported in mammalian systems, suggesting that the zebrafish liver coupled with the available microarray technology present an excellent in vivo toxicogenomic model for investigating arsenic toxicity. We proposed an in vivo, acute arsenic-induced adaptive response model of the zebrafish liver illustrating the relevance of many transcriptional activities that provide both global and specific information of a coordinated adaptive response to arsenic in the liver. microarray expression profiling; arsenic toxicity; oxidative stress; fish toxicogenomics ARSENIC IS AN IMPORTANT and ubiquitous environmental toxicant, and the risk of arsenic poisoning in humans is a public health issue worldwide (1,23,27,46,54). In addition, arsenic is classified as a human carcinogen based on several epidemiological studies showing an association of arsenic exposure with cancers in lung, bladder, kidney, and liver (33, 54). Aside from cancers, arsenic ingestion is also associated with other human diseases such as blackfoot disease, atherosclerosis, hypertension, diabetes mellitus, skin lesions, and liver injury (19,20,37,54,62). Despite these findings and the fact that arsenic is the most extensively studied of the metals and metalloids in drinking water, the molecular mechanisms of arsenic toxicity and carcinogenicity are poorly understood (34,46). This is partly due to the difficulty of inducing cancer with arsenic in animal models and the complex effects induced by arsenic through a variety of mechanisms that influence numerous signal transduction pathways, as these effects vary depending on cell type, dosage, and form of arsenic used (7, 60). Thus many different modes of action have been proposed, and these include both genotoxic and nongenotoxic effects (33,34,46). Some of the genotoxic effects include induction of chromosomal abnor...