In the present study, both zebrafish and medaka vitellogenin genes have been isolated and used as a biomarker to compare the two small aquarium fish in response to estrogen treatment and thus to evaluate the two fish models in development of a biomonitoring system for environmental estrogens. The isolated zebrafish vitellogenin gene, zvtg1, is the most abundantly expressed vitellogenin gene in zebrafish and its complete protein sequence of 1360 amino acids was deduced from a genomic and a cDNA clone. The isolated medaka vitellogenin (mvtg1) genomic clone covers 1053 amino acids in the N-terminal. Both zebrafish zvtg1 and medaka mvtg1 are specifically expressed in female liver and their expression can be induced by 17beta-estradiol (E2) in male fish both by intramuscular injection and immersion treatment. A real-time reverse transcriptase polymerase chain reaction (RT-PCR) assay was developed for quantification of vitellogenin mRNA level in both control fish and E2-treated fish. The lowest-observed-effect concentrations of E2 for the induction of vitellogenin mRNAs were observed at 1 microg/l for zebrafish and 0.1 microg/l for medaka in a 2-day exposure experiment. Further kinetics studies of the two fish models indicated that medaka was able to respond much faster to E2 treatment than zebrafish, while the zebrafish can attain a much higher level of vitellogenin mRNAs than medaka after a long-term E2 treatment. The implication of these observations may be that the medaka system is better in monitoring acute treatment while the zebrafish system is better in monitoring chronic exposure.