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 minium 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+3-2)¿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.