A two-dimensional photochemical model of the atmosphere has been used to calculate dynamical and chemical budgets in the troposphere. Model distributions of O3 and NOy species are compared with observations. The seasonal variations of these species can be explained in terms of the variation of the different components of their budgets. Regional variations are apparent. In the northern hemisphere, where results represent a ''mean'' polluted troposphere, the O3 budget is essentially a balance between net photochemical production and loss via dry deposition in the summer. The flux from the stratosphere plays an important role in the winter and early spring. In the southern hemisphere the flux of O3 from the stratosphere again makes a significant contribution in the winter and spring. The chemical production is much reduced compared with the northern hemisphere. The NOy budget is a balance between NOx emissions and loss via wet and dry deposition of HNO3. ¿ American Geophysical Union 1993