ABSTRACT Mining is one of the most important activities in the economic development of several countries; however, it generates significant impacts on water resources, releasing metals in these environments. The exposure of fish to metals alters the redox capacity of the organism, increasing the production of reactive oxygen and nitrogen species, causing molecular and cellular changes. In 2019, the Paraopeba River basin, an important river in the Neotropical region, suffered from the discharge of tons of mining tailings, affecting the entire aquatic ecosystem. The present study analyzed cellular changes and their relationship with alterations in iNOS and metallothionein levels, caused by the accumulation of metals in the gonads of Hoplias malabaricus . Ten specimens were collected in a control environment and 10 specimens in contaminated Paraopeba River. For histopathological analysis, gonadal fragments were collected from all individuals and submitted to routine histological techniques. Additionally, metallothionein and iNOS were immunolabeled to evaluate oxidative stress and metals' influence. The results showed that several metals in the Paraopeba River led to histopathological alterations in the gonads of males and females. Furthermore, intense positive staining of metallothionein and iNOS in different reproductive cells was demonstrated. This study shows the effects of oxidative capacity caused by metals on the reproductive morphophysiology of fish from the Paraopeba River, even after 5 years since the dam rupture.