To determine whether variations in high-temperature (HT) tolerance in three rice (Oryza sativa L.) cultivars and two N treatments are related to leaf transpiration rate (E), and whether the involvement of nonstructural carbohydrates (NSC) in HT tolerance is related to E, a pot experiment supplied with two N levels (low N, 0.077g urea kg-1 soil; sufficient N, 0.538g urea kg-1 soil) was conducted under ambient temperature (AT) and HT with three cultivars, N22, Zhenshan 97B and Koshihikari. HT significantly decreased grain yield and seed setting percentage in Koshihikari and ZS97, which could be partly offset by a sufficient N supply. The most HT-tolerant cultivar, N22, had the highest E and stem NSC concentrations under both N treatments, whereas the most sensitive cultivar, Koshihikari, had the lowest E and stem NSC concentrations. A sufficient N supply significantly increased E in the three cultivars under the HT treatment. Grain yield and seed-setting percentage were positively related to E and plant NSC concentration under HT, and E was positively related to NSC concentration under both AT and HT. Therefore, variations in HT tolerance among rice cultivars and nitrogen treatments were related to E, and possibly to NSC concentration.