A numerical study of the tropospheric ozone budget is conducted by using a one-dimensional model which takes into account both diffusion and photochemical processes. It is shown that if ozone is treated as an inert species in the troposphere, large vertical eddy diffusion coefficients and an ozone flux from the stratosphere more than twice that given by Fabian and Pruchniewicz (1976) and Danielsen and Mohnen (1977) must be used to reproduce the observed tropospheric ozone profiles in the northern hemisphere mid-latitudes during the summer. Furthermore, consideration of the reactions which destroy ozone in the troposphere would lead to the destruction of about half of the ozone injected in the troposphere. Thus it is likely that there are also reactions which produce ozone in the troposphere, and we examine the possibility that methane oxidation reactions are responsible for this production (Crutzen, 1974a). Indeed, we show that a model which includes both the aforementioned photochemistry and prescribed physical processes can reproduce the observations reasonably; however, the uncertainty of certain reaction rate constants and the lack of understanding of the tropospheric odd nitrogen budget prohibit the quantification of the tropospheric ozone budget. On the other hand, the model results suggest that the time scales of both the physical and photochemical processes are both of the order of 1–3 months. It is therefore certainly not easy to infer the effects of this slow, but ever present, photochemistry from ozone measurements, and it is understandable why it has gone undetected previously.