This study explores the evolution and distribution of carbon monoxide (CO) using the National Oceanic and Atmospheric Administration (NOAA) Geophysical Fluid Dynamics Laboratory three-dimensional global chemical transport model (GFDL GCTM). The work aims to gain an improved understanding of the global carbon monoxide budget, specifically focusing on the contribution of each of the four source terms to the seasonal variability of CO. The sum of all CO sources in the model is 2.5 Pg CO/yr (1 Pg=103 Tg), including fossil fuel use (300 Tg CO/yr), biomass burning (748 Tg CO/yr), oxidation of biogenic hydrocarbons (683 Tg CO/yr), and methane oxidation (760 Tg CO/yr). The main sink for CO is destruction by the hydroxyl radical, and we assume a hydroxyl distribution based on three-dimensional monthly varying fields given by Spivakovsky et al. <1990>, but we increase this field by 15% uniformly to agree with a methyl chloroform lifetime of 4.8 years <Prinn et al., 1995>. Our simulation produces a carbon monoxide field that agrees well with available measurements from the NOAA/Climate Monitoring and Diagnostics Laboratory global cooperative flask sampling network and from the Jungfraujoch observing station of the Swiss Federal Laboratories for Materials Testing and Research (EMPA) (93% of seasonal-average data points agree within ¿25%) and flight data from measurement campaigns of the NASA Global Tropospheric Experiment (79% of regional-average data points agree within ¿25%). For all 34 ground-based measurement sites we have calculated the percentage contribution of each CO source term to the total model-simulated distribution and examined how these contributions vary seasonally due to transport, changes in OH concentration, and seasonality of emission sources. CO from all four sources contributes to the total magnitude of CO in all regions. Seasonality, however, is usually governed by the transport and destruction by OH of CO emitted by fossil fuel and/or biomass burning. The sensitivity to the hydroxyl field varies spatially, with a 30% increase in OH yielding decreases in CO ranging from 4--23%, with lower sensitivities near emission regions where advection acts as a strong local sink. The lifetime of CO varies from 10 days over summer continental regions to well over a year at the winter poles, where we define lifetime as the turnover time in the troposphere due to reaction with OH. ¿ 2000 American Geophysical Union