Fish gills are target organs for waterborne metal ions and this work aimed to investigate the effects of waterborne Ni²⁺ (10, 25 and 50mgL⁻¹) on goldfish gills. A special focus was on the relationship between Ni uptake and the homeostasis of reactive oxygen species (ROS) in the gills, the tissue, in direct contact with the metal pollutant. Ni-accumulation in the gills occurred as a function of exposure concentration (R²=0.98). The main indices of oxidative stress, namely carbonyl proteins (CP) and lipid peroxides (LOOH), decreased by 21–33% and 21–24%, as well as the activities of principal antioxidant enzymes superoxide dismutase and glutathione-dependent peroxidase, by 29–47% and 41–46%, respectively, in gills of Ni-exposed fish. One of the main players in the antioxidant defense of gills seems to be catalase, which increased by 23–53% in Ni-treated fish, and low molecular mass thiol-containing compounds (L-SH), exceeding untreated controls by 73–105% after fish exposure to 10–50mgL⁻¹ of Ni²⁺. The increased level of L-SH, mainly represented by reduced glutathione, was supported by enhanced activities of glutathione reductase (by 27–38%), glutathione-S-transferase (56–141%) and glucose-6-phosphate dehydrogenase (by 96–117%) and demonstrates the ability of the antioxidant system of gills to resist Ni-induced oxidative stress.