Histories of atmospheric N2O concentration and its δ 15N and δ 18O were reconstructed for the period 1952--2001 on the basis of the analyses of firn air collected at the North Greenland Ice Core Project (NGRIP), Greenland, and Dome Fuji and H72, Antarctica. The N2O concentration increased from 290 ppbv in 1952 to 316 ppbv in 2001, which agrees well with the results from atmospheric observations and polar ice core analyses. The δ 15N and δ 18O showed a secular decrease, the respective values being 8.9 and 21.5? in 1952 and 7.0 and 20.5? in 2001. Their rates of change also varied, from about -0.02? yr-1 in the 1950s to about -0.04? yr-1 in 1960--2001 for δ 15N, and from about 0? yr-1 to -0.02? yr-1 for δ 18O. The isotopic budgetary calculations using a two-box model indicated that anthropogenic N2O emission from soils played a main role in the atmospheric N2O increase after industrialization, as well as that the average isotopic ratio of anthropogenic N2O has potentially been changed temporally.