The time- and dose-dependent transcriptional and translational expression of biomarker genes in nonylphenol (NP) and estradiol-17beta (E(2)) treated juvenile rainbow trout is reported. Fish were exposed to NP (1, 5 and 25 mg/kg) and E(2) (5 mg/kg) and killed at 2, 6, 12, 24, 48 and 72 h after exposure. The estrogen receptor (ER), vitellogenin (Vtg) and eggshell zona radiata protein (Zr-protein) gene expressions were analyzed in total liver RNA using Northern and slot hybridization with specific cDNA probes. Plasma Vtg and Zr-protein levels were evaluated using indirect ELISA. While Zr-protein gene showed an induction only at 24 h post-exposure, the plasma protein levels showed a time-dependent increase in the 25-mg NP treated group. Vtg transcripts showed an apparent time-dependent increase without a concomitant increase in protein levels in the 25-mg NP treated fish. Time-dependent increases in Vtg and Zr-protein gene expressions without the corresponding increases in ER gene transcription was observed in E(2)-treated fish at 2, 6 and 12 h post-exposure. Induction of ER gene transcripts was observed from 24 h and did not change significantly at 48 and 72 h. In the E(2)-treated fish, induction of plasma Vtg levels was observed at 48 and 72 h, while plasma Zr-protein was induced at 24, 48 and 72 h, after exposure. We conclude that the E(2)- and NP-induced Vtg and Zr-protein gene expressions at the early time intervals after exposure are not dependent on increase in the transcriptional activity of the ER gene and that Vtg and Zr-protein gene transcriptions require only basal or minimal ER concentration, in addition to other mechanisms.