Nonylphenol, a chemical widely utilized in the productuion of surfactants and pesticides, could interfere with the endocrine system of organisms. Large amount of this compound is released into the aquatic environments through industrial, agricultural and municipal effluents. The objective of the present study was to investigate the potential effects of a single-dose of nonylphenol at environmental level on the balance of thyroid and pituitary hormones of juvenile Caspian brown trout (Salmo trutta caspius). For this, fish with average weight of 40±10g was exposed for 21 days to a single concentration of 30µg/L nonylphenol and then, the plasma levels of thyroid (T3 and T4) and pituitary (TSH, FSH and GH) hormones, as well as GSI and HSI were studied in both male and female fish. The results showed that nonylphenol at environmentally relevant concentration had significant effects on the levels of GH, T3 and T4 hormones compared with control group. In addition, male and female fish showed different sensitivity in response to nonylphenol exposure. Regarding to the disruptive effects of environmental level of nonylphenol on juvenile Caspian trout, authorities must take necessary measures to prevent the entry of such chemicals into the Caspian Sea, as well as to protect the endangered fishes from mentioned chemicals, and to choose appropriate contaminant-free areas for releasing the fish.