The global budget of N sub 2 O shows a significant imbalance between the known rate of destruction in the stratosphere and the estimated rates of natural and anthropogenic production in soils and the ocean.Measurements of the15 N/sup 14 N and18 O/sup 16 O ratios in two major tropospheric sources of N2 O, tropical rain forest soils and fertilized soils, show that soil N sub 2 O from a tropical rain forest in Costa Rica and from sugar-cane fields in Maui is strongly depleted in both15 N and sup 18 O relative to mean tropospheric N2 O. A major source of heavy N2 O, enriched in both15 N and18 O, must therefore be present to balance the light N2 O from soils. One such source is the back-mixing flux of N2 O from the stratosphere, which is enriched in15 N and18 O by photolysis and chemistry. However these return fluxes of15 N and18 O are so great that a large oceanic flux of N2 O is required to balance the heavy isotope-enriched stratospheric flux. All these effects will be reflected in climatically related isotopic variations in trapped N2 O in polar ice cores.