Abstract. Abstract: Aim Paracetamol (PCM), or acetaminophen, is one of the most used drugs for human treatment of pain and fever. Since it has often been found in the aquatic environment, the aim of this study was to investigate the effects of PCM on the reproductive axis of male Rhamdia quelen catfish. Methods Different biomarkers were evaluated in hypothalamus, liver and gonads, as well as the plasma sexual hormone quantification. The fish were exposed to three PCM concentrations: 0.25, 2.5 and 25 µg.L-1 and to a control group (solvent acetone 0.0003%). After 14 days of exposure, fish were anesthetized, for blood sampling and biometrics, and after euthanasia, the tissues were sampled. Plasma was used for 11- keto testosterone and 17β - estradiol quantification. The hypothalamus was collected for brain aromatase (cyp19a1b) gene expression; the liver for the vitellogenin (vtg) gene expression and biochemical biomarkers; and gonad for the biochemical and histological biomarkers analyses. Results No alterations were observed in the hormone’s levels, sexual maturation level or in cyp19a1b and vtg gene expression. In the liver the non-protein thiols concentration increased at 2.5 µg.L-1 of PCM, and the superoxide dismutase (SOD) activity was reduced at 0.25 µg.L-1 of PCM. In gonads, glutathione S-transferase (GST) activity decreased and SOD activity increased at 25 µg.L-1 of PCM, while the glutathione peroxidase (GPx) activity reduced after exposure to both PCM concentrations. Conclusion The results showed that environmental concentrations of PCM can cause alterations in the antioxidant system, mainly in the gonads of R. quelen males. However, without significant change in the hormones levels or in the expression of genes related to the reproduction axis. These alterations occurred at concentrations already found in aquatic environment, including in Brazil.