The budget fo tropospheric ozone is reexamined in the light of lower observed nitrogen oxide densities and revised reaction rates. To estimate the relative importance of transport and photochemistry, model calculations are presented which determine one-dimensional vertical ozone profiles at mid-latitudes for equinoctial conditions. These calculations imply that both photochemistry and transport are important in controlling the abundance of tropospheric ozone. We estimate that photochemical processes produce about 0.6¿1011 and destroy about 1.4¿1011 tropospheric ozone molecules cm2 s1, while mixing processes transport about 1.4¿1011 ozone molecules cm2 s1 from the stratosphere into the troposphere and ground levle fluxes destroy about 0.6¿1011 ozone molecules cm2 s1. The sensitivity of our model calculations to transport parameters, key rate coefficients, and nitrogen oxide densities are discussed.