The toxic impact of environmentally relevant nominal sub-lethal concentration of lead nitrate (23 mg/L) on the gills of Cyprinus carpio after 30 days of exposure was assessed. Structural alterations were analyzed through histopathology, and the DNA damage rate in peripheral erythrocytes was evaluated by alkaline comet assay. A significant deviation in the gill histoarchitecture was observed compared to the control group. Significant changes, including the curling of secondary lamellae, loss of filaments, necrosis, hyperplasia, hypertrophy of cells of primary filament, and vacuolization, were found in the fish exposed to nominal sub-lethal concentration of lead nitrate. Moreover, the comet assay showed lead nitrate-induced DNA damage, evidenced by the length of the DNA "tail" in the exposed fish. The findings of this study strongly indicated that lead nitrate, even at sub-lethal levels, could significantly alter the overall physiology of the fish. This situation could lead to severe ecological consequences.