In order to monitor potential contamination deriving from exploration and transport of oil in the Urucu region (Brazil), there is a need to establish suitable biomarkers for native Amazonian fish. Therefore. the transcript expression of various potentially sensitive genes (ahr2(1), cyp1a, hmox1, hsp70, malt, mt. nfe212,gstp1 and nqo1)in fish exposed to water soluble fractions of oil (WSF) was compared. The analysis was first performed in an established laboratory model, the zebrafish embryo. The cyp1a gene proved to be the most sensitive and robust marker for oil contamination and, hence, was selected to study the effect of oil-derived contaminants in the Amazonian cichlid Astronotus ocellatus. Induction of cyp1a transcript expression was observed for >= 0.0061% (v/v) WSFs. In liver samples of fish, collected from different lakes in the Urucu oil mining area, no elevated expression of cyp1a transcripts was observed. The data demonstrate the high sensitivity of cyp1a as indicator of oil exposure: further studies should be considered to test its usefulness at known contaminated sites and to evaluate influential factors by, e.g. mesocosm experiments. (C) 2011 Elsevier B.V. All rights reserved.