Brain estrogen production, performed by the enzyme aromatase, can be disrupted/affected in teleost fish exposed to endocrine disruptors found in polluted aquatic environments. The guppy (Poecilia reticulata) was previously studied and confirmed to suffer negative effects on reproductive behaviors following inhibition of the brain aromatase reaction. Here adult guppies (Poecilia reticulata) of both genders were subjected to known endocrine disruptors: the androgen androstenedione (A), the synthetic estrogen 17 alpha-ethinylestradiol (EE2), and the estrogenic surfactant 4-nonylphenol (NP), at high (50 mu g/L) and at environmentally relevant concentrations (10 ng/L EE2, 5 mu g/L NP, and 0.7 mu g/L A) for 2 weeks followed by measurements of brain aromatase activity (bAA). In the adult males, bAA was stimulated by A and EE2 at 50 mu g/L. Female activity was also stimulated by the higher estrogenic treatment. At environmentally relevant concentrations only the EE2 treatment affected bAA, and only in males. The alkylphenolic substance NP produced no effect in either of the experiments, not on males nor females. The results indicate that short-term steroid treatments have stimulatory effects on guppy brain aromatase even at concentrations that can be found in the environment. We thus suggest bAA of adult guppies to be a suitable bioindicator of endocrine disruptors.