The synthetic organic compound 4-nonylphenol (4-NP) has been shown to have a wide range of adverse effects on the endocrine system of various animals including fish. The present study evaluated the potential effects of 4-NP on vitellogenin (VTG) synthesis, steroid, and thyroid hormone concentrations in both juvenile male and female rainbow trout (Oncorhynchus mykiss). Fish were exposed by intraperitoneal injection to different doses of 4-NP (1, 10, 50, and 100 μg g(-1)) or vehicle (coconut oil) over a period of 14 days. Blood samples were collected 7 and 14 days after initiation of treatment. Plasma VTG levels in 4-NP-treated fish were detected by sodium dodecyl sulfate polyacrylamide gel electrophoresis as a high molecular weight protein band of 180 KDa. In addition, plasma VTG concentrations were quantified indirectly using plasma alkali-labile phosphate (ALP) and plasma calcium. Both ALP and calcium levels in plasma showed similar and parallel increase patterns after exposure to 4-NP that were significantly higher compared with controls. The analysis of plasma sex steroid levels revealed a significant increase in 17β-estradiol and testosterone in plasma of juvenile males and females, respectively. Furthermore, a significant increase was observed in plasma cortisol levels. On the other hand, 4-NP decreased both plasma triiodothyronine and thyroxine after 7 and 14 days of treatment. These results suggest that 4-NP can affect different parts of the endocrine system, which may lead to serious impairments in physiological homeostasis of juvenile rainbow trout.