In situ measurements of ozone (O3) and carbon monoxide (CO) were conducted using a passenger aircraft during 47 flights (November 1997 to April 2001) between Germany and the Indian Ocean region at 10--11 km altitude, project Civil Aircraft for Regular Investigation of the Atmosphere Based on an Instrument Container (CARIBIC). Here, emphasis is on a better understanding of the budgets of O3 and CO in the middle and upper northern hemispheric (NH) troposphere. Latitudinal and seasonal variations of the mean values and the local variability of O3 and CO are assessed. CO is moreover applied as tracer of polluted (surface) air, that is, as a measure of O3 precursors. This enables us to assign positive or negative O3--CO correlations and their slopes α = dO3/dCO to O3 produced by photochemistry or to O3 imported from the stratosphere. In the tropics, photochemical O3 production clearly dominates all year round, with mean O3--CO slopes of 0.45--0.53 in spring/summer and 0.20--0.27 in autumn/winter. Over the Arabian Sea and the Middle East, high annual net O3 production rates in the troposphere of 17.6 ¿ 1010 O3 molecules cm-2 s-1 are estimated. In the extratropics, the well-documented springtime O3 maximum emerged in the CARIBIC data set. Stratospheric O3 influx and photochemical O3 production contribute equally to this spring maximum. In summer, photochemical O3 formation clearly dominates, in winter, per contra, stratospheric O3 influx clearly dominates. In the extratropics, CO shows a sine seasonal variation between ~81 ppbv in September and ~111 ppbv in March, being delayed by ~5 weeks compared to ground-based background monitoring stations. In the tropics, however, no significant seasonal variations of CO were observed, mainly because of occasional high CO periods such as those associated with the 1997 Indonesian forest fires or the outflow of the Intertropical Convergence Zone, which pumps in summer immense amounts of pollutants emitted in South Asia (India, Bangladesh, Indochina) to the 10--11 km altitude range surveyed during CARIBIC.