Ground-based measurements of the concentration of 14CO are reported. The data exhibit a well-defined seasonal variation at mid-latitudes with a maximum of 25 ± 2 molecule cm−3 during winter and a minimum of 11 ± 1 molecule cm−3 during summer. The measurements also indicate a strong latitudinal variation, with the lower concentrations occurring in the tropics. The data are interpreted using a 2-D time-dependent model. To balance the sources of both 14CO and 12CO, an average tropospheric OH concentration of (6.5-2+3) × 105 molecule cm−3 is required. In addition, the 12CO production rate from biological processes including the oxidation of nonmethane hydrocarbons is found to be 1250 × 1012 g CO yr−1.