The knowledge of critical sites of interaction between pollutants and fish is a major means of investigation in order to obtain early advance knowledge of adverse alterations and ecotoxicological risks. In fish, the ovarian secretion of oestradiol-17beta has been demonstrated to govern hepatic synthesis of vitellogenin, a lipoprotein corresponding to the major part of embryonic trophic reserves. Oestradiol synthesis is catalysed by aromatase, a cytochrome P450-dependent enzymatic activity. Effects of different imidazole fungicides, well known as potent inhibitors of cytochrome P450s, were tested in vitro. A concentration-dependent inhibition of the microsomal aromatase was observed with an inhibiting potency order as follows: clotrimazole (Inhibition 50%: 5.10(-7) m) > prochloraz and imazalil (I 50%: 5.10(-6)M) > ketoconazole (I 50%: 5.10(-4)). Secretion of oestradiol from cultured follicles was reduced after exposure to 10(-6)M prochloraz in the culture medium.