Hormonally active compounds of different origins such as polychlorinated biphenyls (PCBs) and their derivatives are widely dispersed in the environment by various industrial processes. They represent a threat to human health since their toxic effects can result from biological accumulation of low doses of active compound during extended periods of exposure. Critical doses for a cytotoxic effect have been determined using cellular models such as cultured hepatocytes but it is still not known if lower doses can affect in vitro cell functions before any toxic effect can be detected. To address this issue we have used a functional genomic approach to characterize changes in the profile of genes expressed by cultured bovine fibroblasts exposed during only 26 hours to concentrations as low as 10-10 to 10-12 molar of TCDD