To obtain basic information on the critical levels of O₃, for the protection of Japanese coniferous forests, the growth responses of Larix kaempferi, Pinus densiflora and Cryptomeria japonica seedlings to O₃ under different nitrogen (N) loads were investigated. The seedlings were grown in 12 experimental treatments, which were comprised of four gas treatments (charcoal-filtered air and three levels of O₃ at 1.0, 1.5 and 2.0 times ambient concentration) in combination with three N treatments (0, 20 and 50 kg ha^-1 year^-1), during the two growing seasons. Although no significant interaction between O₃ and N load was detected for the growth of P. densiflora and C. japonica seedlings, significant interaction between O₃ and N load for the growth of L. kaempferi seedlings was found at the end of the second growing season. The N load reduced the sensitivity of growth of L. kaempferi seedlings to O₃. Critical levels of O₃ expressed as AOT40 for one growing season from April to September, corresponding to a 10% reduction in the whole-plant dry mass increment, were evaluated to be 28 (NO), 29 (N20) and 53 (N50) μmol mol^-1 h in L. kaempferi, 38 μmol mol^-1 h in P. densiflora and 111 μmol mot^-1 h in C. japonica. In the three tree species, a high correlation was found between the sensitivity to O₃ of the whole-plant dry mass increment and that of the total net photosynthetic rate (A_total), expressed as the product of needle dry mass and net photosynthetic rate in the summer of the second growing season. It was concluded that N deposition must be taken into account in the evaluation of the critical level of O₃ for the protection of L. kaempferi forests. The A_total can be used as an indexical plant factor for estimating the sensitivity to O₃ of Japanese coniferous forest tree species under different N loads.