Free-air O₃ enrichment was used to investigate the responses of different antioxidant mechanisms in different rice (Oryza sativa L.) cultivars - O₃-sensitive hybrid indica (O₃-S) cultivars and O₃-tolerant conventional japonica (O₃-T) cultivars across all growth stages. Elevated [O₃] induced increases in reactive oxygen species (ROS) production in O₃-S cultivars, which were more pronounced in the later growing stages. In O₃-S cultivars, continuous O₃ stress decreased catalase (CAT), peroxidase (POD) and glutathione peroxidase (GPX) activities, while in O₃-T cultivars, short-term O₃ stress decreased superoxide dismutase (SOD), CAT, POD and GPX activities. The same POD isozyme patterns were observed in both O₃-S and O₃-T cultivars, while SOD and APX isozymes varied by cultivar. The results suggest that O₃ tolerance might be improved at different rice development stages through regulating the responses of antioxidant mechanisms to O₃ stress.