Fluctuations in the concentrations of stratospheric trace gases are often correlated over a large range of space and time scales, an observation frequently used to infer the existence of various chemical processes. Three-dimensional models provide a tool to examine the causes and variations of trace gas relationships, because they can realistically simulate the interplay between stratospheric photochemistry and meteorology. Thus such models can aid the interpretation of observed trace gas relationships. We use the general circulation model of the Goddard Institute for Space Studies to simulate the evolution and distribution of N2O, CO2, and O3 over a year. In the modeled lower stratosphere the constituents N2O and CO2 have well-correlated spatial variations, but the slope of the regression line depends on both the season and the direction of sampling. This departure from a universal form is due both to the annual cycle in tropospheric CO2 and to transport of air from the upper stratosphere photochemically depleted in N2O. Due to the short photochemical lifetime of tropical O3, its relationship with N2O is still more varied. In particular, the slope of the O3--N2O regression line changes significantly from middle to high latitudes, behavior relevant to the use of N2O for estimating the rate of polar winter O3 depletion. In general, a tight correlation between two trace gases such as N2O and O3 is often observed, but this datum cannot be used to infer a similar universal relationship because a different direction of sampling may change the slope and the scatter about it. ¿ American Geophysical Union 1995