###### Abstract  
The physiological conditions of an organism may influence its ability to cope with environmental stressors, such as contaminants. Biotransformation and the endocrine system interact with each other to promote animal's fitness. However, little is known regarding the interaction between hormones and response to pollutants such as polycyclic aromatic hydrocarbons (PAHs). In this _in vitro_ study, we aimed to increase the knowledge regarding the effects of steroid hormones on ethoxyresorufin-O-deethylase (EROD) activity inhibited by contaminants. The effects on _in vivo_ induced EROD activity of _Anguilla anguilla_ were assessed by conducting single and combined exposures to fluoranthene (FL) and to physiological levels of two major steroid hormones (cortisol and 178-estradiol). Hepatic microsome exposure to the lowest concentrations of FL (0.1 and 0.3 uM), as well as to cortisol and 178-estradiol (E\({}_{2}\)), led to significant EROD activity induction. However, the highest tested concentrations of FL (0.9 and 2.7 uM) significantly inhibited this enzymatic activity. When microsomes were simultaneously exposed to 0.9 uM FL and one of the hormones, both cortisol and E\({}_{2}\) were able to decrease the inhibitory effects, with the former completely reverting EROD activity inhibition. These findings support the idea that cortisol and E\({}_{2}\) can help prevent the inhibitory effects of PAHs over biotransformation enzymes, highlighting the physiological relevance of these hormones. organic contaminants; endocrine regulation; biotransformation enzymes; _in vitro_ effects 20222123098222309822309822309822309822309823098230982309823098230982309823098230982309823098  
aryl hydrocarbon receptor (AhR) transcription factor, which is involved in the regulation of the genes that code for the enzymes involved in biotransformation [7]. EROD activity is frequently used as a biomarker of exposure to organic contaminants such as PAHs and structurally related compounds (e.g., \(\delta\)-naphthoflavone (BNF)), which have been used to induce EROD activity in several studies [3; 9; 10]. However, EROD activity may be influenced in the natural environment by, for example, organisms' endogenous factors, including physiological status and endocrine levels [11]. Hormones play an essential role in CYP1A expression, either directly by influencing CYP1A gene expression or AhR function or indirectly via crosstalks with other transcription factors [12]. The physiological condition of a given organism may influence its ability to respond to environmental stressors (e.g., contaminants). The endocrine system plays an important role in animal fitness because exposure to pollutants often results in an endocrine stress response, which ensures the organism's survival by helping restore homeostasis. Primary stress, such as short-term exposure to contaminants, stimulates the hypothalamic-pituitary-inter-renal (HPI) axis, leading to the production of cortisol, a major stress hormone in fish that mobilizes energy required by increased metabolic rates [13; 14; 15]. Cortisol, the main glucocorticoid in fish, is also a key hormone for behavioral responses to recognized threats (e.g., predation threat) [16]. 178-estradiol (E\({}_{2}\)), the most potent estrogen, is a key steroid hormone in both sexes, involved in growth, development, morphological differentiation, sexual behaviors, and cycles [17]. Thus, biotransformation and the endocrine system are expected to interact to promote the animal's fitness. Steroid hormones have the ability to cross the hepatocyte's cell membrane and may have a direct effect on microsomes. However, in fish, the relation between EROD activity and plasmatic and cellular levels of certain steroid hormones, such as cortisol and E\({}_{2}\), is poorly known. Such knowledge may be vital to a better understanding of the potential link between endocrine regulation, stress response to pollutants, and the ability to metabolize them.