Results from a tropospheric three-dimensional chemical transport model (TOMCAT) have been used to examine the terms of the ozone budget, both regionally and globally. The global calculations are discussed in light of other published estimates. Regional budgets are calculated for continental regions, including the American Mid-West, Sahara, and central Europe. These are compared with regional budgets for oceanic regions, including the Azores High and the Tropical Pacific Warm Pool. Furthermore, the coastal region of the UK and Ireland is also considered. The validity of these regional budgets from TOMCAT are discussed by comparing TOMCAT with measurements from a number of aircraft campaigns. The budgets for central Europe and the American Mid-West indicate that continental regions dominate the ozone budget of the northern extratropics. This is in spite of the remote oceanic regions being photochemical sinks for ozone. The regional budget calculations for the UK and Ireland exhibit net photochemical production of ozone in the boundary layer but this is not consistent with available aircraft measurements. This is attributed to the coarse horizontal resolution of the TOMCAT model which results in the model's photochemical budget being more typical of a polluted continental region than a relatively remote one. On the other hand, the ozone photochemical rates calculated for the Azores High and the Tropical Pacific Warm Pool agree rather well with other estimates.