Increasing the yield of rice per unit area is important because of the demand from the growing human population in Asia. A group of varieties called erect panicle-type rice (EP) achieves very high yields under conditions of high nitrogen availability. Little is known, however, regarding the leaf photosynthetic capacity of EP, which may be one of the physiological causes of high yield. We analyzed the factors contributing to leaf photosynthetic rate (P-n) and leaf mesophyll anatomy of Nipponbare, Takanari, and Shennong265 (a EP type rice cultivar) varieties subjected to different nitrogen treatments. In the field experiment, P-n of Shennong265 was 33.8 mu mol m(-2) s(-1) in the high-N treatment, and was higher than that of the other two cultivars because of its high leaf nitrogen content (LNC) and a large number of mesophyll cells between the small vascular bundles per unit length. In Takanari, the relatively high value of P-n (31.5 mu mol m(-2) s(-1)) was caused by the high stomatal conductance (g s;.72 mol m(-2) s(-1)) in the high-N treatment. In the pot experiment, the ratio of P-n/C-i to LNC, which may reflect mesophyll conductance (g(m)), was 20-30% higher in Nipponbare than in Takanari or Shennong265 in the high N availability treatment. The photosynthetic performance of Shennong265 might be improved by introducing the greater ratio of P-n/C-i to LNC found in Nipponbare and greater stomatal conductance found in Takanari.