Recent in vitro studies have demonstrated that copper may induce apoptosis triggering the activation of caspase-3, a central effector of apoptotic cell death. However, the precise mechanism of copper-induced apoptosis is still unclear, even less so in Oreochromis niloticus where no caspase genes have been reported so far. This study aimed to assess the in vivo role of copper in apoptosis induction on O. niloticus gill, simultaneously contributing to elucidate the mechanism of copper-induced apoptosis. Caspase-3 gene was partially sequenced and, after in vivo exposures to 40 and 400 microgL(-1) of copper, its mRNA expression was evaluated by real-time PCR. Apoptosis was also evaluated by TUNEL assay and cell proliferation identified using an antibody against proliferating cell nuclear antigen (PCNA). The copper concentrations used did not induce the upregulation of caspase-3 gene in O. niloticus gill. In addition, in the gills of fish exposed to copper there was no increase in the estimated relative volume of apoptotic cells, indicating that neither the caspase-3-dependent or caspase-independent apoptotic pathways were induced. On the other hand, the increase in the volumetric density of epithelial proliferating cells suggests a concentration-dependent repair response.