The small-scale vertical variability of tropospheric O3 and CO is examined from a set of simultaneous measurements obtained in July and August 1974, between 55 ¿S and 67 ¿N. From this set of vertical profiles, it is noted that many of the fluctuations are coincident in both species, and a method is presented that quantifies the correlation between the observed O3 and CO variability. A two-dimensional depiction of the distribution of these O3-CO correlations shows that there are regions in the troposphere where these trace gases are positively correlated, while at the same time there are preferred locations where these two species are primarily anticorrelated. The regions of anticorrelation are consistent with the traditional picture of the tropospheric ozone cycle, which suggests that this gas is chemically unreactive in the troposphere. On the other hand, the location and magnitude of the region in which these two species are positively correlated suggest that there is considerable in situ production of tropospheric ozone, which is likewise consistent with the more recent interpretation of the tropospheric ozone cycle that shows that this gas is photochemically active in the troposphere. In addition, the existence of a significant in situ source of tropospheric O3 in the northern hemisphere is supported by a modeling study which likewise suggests that the observed hemispheric asymmetry in the distribution of tropospheric O3 is quite likely a result of surface emissions of precursors.