The effects of the estrogenic compound 4-nonylphenol (4-NP) on vitellogenesis in juvenile olive flounder ( ) and rockfish ( ) exposed continuously at 10, 50 and levels for 7 days were compared. The expression of VTG mRNA level and protein using specific probes were examined. The levels of plasma estradiol-17 and testosterone (T) were assessed by radioimmunoassay (RIA). Plasma concentrations increased significantly in two female fish species exposed to of 4-NP over concentrations in control fish. Plasma T concentrations increased in . Four days after exposure, the level of VTG mRNA expression increased in and exposed to of 4-NP. In addition, plasma VTG protein expression was seen in and . In and exposed to 4-NP, the changes were noticed mainly in hepatocytic vaculation after 7 days of exposure. Thus, 4-NP may disrupt vitellogenesis in immature fish both directly and indirectly via disrupted steroidogenesis and liver pathology. Immature were the most sensitive to 4-NP exposure in vitellogenesis.