Effects of nitrogen (N)-deficiency on midday photoinhibition in flag leaves were compared between two contrastive Japanese rice cultivars, a traditional japonica cultivar with low yield, cv. Shirobeniya (SRB), and a japonica-indica intermediate type with high yield, cv. Akenohoshi (AKN). Both cultivars were grown under high-N and low-N conditions. At midday, low-N supply resulted in more intensive reductions in net photosynthetic rate, stomatal conductance, maximal quantum yield of photosystem II (PSII) and quantum yield of PSII electron transport in SRB than in AKN, indicating that SRB was more strongly photoinhibited than AKN under low-N condition. At midday, the low-N plants of two cultivars showed higher superoxide dismutase (SOD) activities than the high-N plants. However, ascorbate peroxidase (APX) activity was maintained in AKN but significantly decreased in SRB under low-N condition (N-deficiency). In contrast, hydrogen peroxide (H2O2) content in SRB significantly increased under low-N condition, indicating that the susceptibility to midday photoinhibition in the low-N plants of SRB is related to the increased H2O2 accumulation. It is suggested that the midday depression in photosynthesis may be a result of oxidative stress occurring in the low-N plants in which antioxidant capacity is not enough to cope with the generation of H2O2. Therefore, H2O2-scavenging capacity could be an important factor in determining the cultivar difference of midday photoinhibition in flag leaves of rice under low-N condition. and E. Kumagai, T. Araki, O. Ueno.