The aim of the study was to investigate the effects of subchronic exposure of zebrafish (Danio rerio) to a fluoroquinolone norfloxacin, using selected oxidative stress parameters as a target. Toxicity tests were performed on zebrafish according to the OECD Guidelines number 203 and number 215. In the Subchronic Toxicity Test, a significant (P < 0.01) increase in the activity of glutathione peroxidase, glutathione S-transferase, and catalase was found. In the test, norfloxacin did not affect lipid peroxidation and catalytic activity of glutathione reductase. From the results, we can conclude that norfloxacin has a negative impact on specific biochemical processes connected with the production of reactive oxygen species in fish tested.