The presence of the female-specific yolk protein precursor vitellogenin in blood and liver from male fish is widely used as an indicator of endocrine disruption. We studied the induction of vitellogenin mRNA in liver from several species of fish, both maintained in fish tanks or captured in the wild. Our procedure requires minute amounts of liver samples (down to 50 mg), and can be applied to field samples if the appropriate RNA-stabilisation agent is used. We used reverse-transcriptase PCR and quantitative real-time PCR for detection and precise quantitation of vitellogenin mRNA levels. Male mummichog (Fundulus heteroclitus) exposed to 17 beta-estradiol contained levels of vitellogenin mRNA up to 30 times higher than in untreated females and treatment with nonylphenol resulted in a weak but consistent induction of this transcript. We also studied levels of vitellogenin mRNA in a population of common carp (Cyprinus carpio) from the Anoia river, a river known for its high levels of estrogenic alkylphenols. The results were consistent with recorded data for fish from this sampling site. Finally, we also detected vitellogenin mRNA in barbs (Barbus graellsi), a species for which no vitellogenin sequence was available. The use of mRNA quantitation techniques for analysis of feral and cultured fish of different species opens the possibility of more precise detection and further control of the noxious effects of contaminants on the local fauna exposed to them.