Estrogenic chemicals have drawn considerable attention over the past several years because they are known to be responsible for increased breast and prostate cancer. Vitellogenin expression driven by estrogen has been becoming a model for studying estrogenic effects in aquatic species. For the first time, we showed evidence that, without stimulation, vitellogenin mRNA precursor is expressed in both male and female immature fish. After 4 h, in fish treated with estradiol, the vitellogenin mRNA was synthesized and the precursor mRNA began to disappear. The environmental chemical, nonylphenol, showed the same effect on the vitellogenin gene expression as estrogen. It is suggested that estrogen and nonylphenol may be involved in a post-transcriptional regulation process -- possibly in the initiation of pre-mRNA splicing.