The authors describe a three-dimensional global photochemical-tran sport model of the troposphere and stratosphere based on the circulation and temperature fields derived from the United Kingdom Meteorological Office data assimilation system. The results of numerical simulation of the space-time distribution of minor,oases are compared to satellite measurements and to the estimates obtained from a number of other three-dimensional photochemical models. This validation shows that the presented model satisfactorily reproduces the space-time distributions of O-3, NOx, HNO3, and ClONO2 mixing ratios, adequately reflects the real situation in the atmosphere, and can be successfully employed for analyzing the factors causing ozone-layer variability.