Gene expression of the biotransformation enzymes, glutathione S-transferase (GST) glutathione peroxidase (GPx) and glutathione synthesizing enzyme glutamyl-cysteine-ligase (GCL), was investigated in zebrafish (Dani orerio) exposed to arsenate, perchlorate, a mixture of these two chemicals, and thyroxine (T4). The exposure persisted for 30 days for perchlorate, 7 days for arsenate and 7 days for thryroxin. To test the mixture toxicity of arsenate and perchlorate, arsenate was added for 7 days in addition to 30 days exposure to perchlorate. Arsenate is known to induce oxidative stress, and GCL, GPx, and GST are involved in the oxidative stress response. Thyroid hormone is involved in mediating the oxidative stress response, and perchlorate is a known thyroid gland disruptor. Therefore, this research was designed to test the hypotheses that 1) thyroid hormone affects the gene expression of GCL, GPx, and GST enzymes and 2) thyroid hormone modulates As-induced expression of these genes. Fish were exposed to the mixture of arsenate and perchlorate to examine the effect of thyroid hormone on As-induced gene expression of GCL, GPx, and GST. The findings indicate that arsenate up-regulates the transcription of GCL and GST pi genes in the gills and GSTpi gene in liver, and this induction was not observed in presence of perchlorate. Therefore, it can be concluded that thyroid hormone inhibits As-induced gene expression of GCL and GSTpi.