A three-dimensional global chemistry transport model is used to examine the impact of stratosphere to troposphere fluxes of ozone (O3) and nitric acid (HNO3) on tropospheric chemistry. The stratospheric fluxes are parameterized as a tropospheric source of O3 and HNO3. The accuracy of the resulting model simulation is compared with measurements. The tropospheric impact of the stratospheric fluxes is examined through a parallel simulation that includes no stratospheric fluxes of O3 and HNO3. Stratosphere-troposphere exchange (STE) increases the global average tropospheric ozone column by only 11.5%, increasing it in the Northern Hemisphere by 10.5% and in the Southern Hemisphere by 13%. STE shifts the springtime ozone maximum ~1 month earlier in the Northern Hemisphere. The portion of the O3 distribution of stratospheric origin in the troposphere increases with altitude, from a maximum of 10--20% in the lower troposphere to 40--50% in the upper troposphere. The sensitivity of the tropospheric response to the spatiotemporal distribution of STE is also examined. On a hemispheric and annual scale the tropospheric composition is particularly sensitive to the temporal distribution of STE. The separate roles of stratospheric fluxes of O3 and HNO3 are also identified. ¿ 1999 American Geophysical Union