The first polar ice core measurements of carbon monoxide compatible with atmospheric data were obtained using an improved experimental protocol. A new CO extraction method has been developed with special care to eliminate any CO contamination. The procedure has been applied to ice core samples originating from Antarctica and Greenland in order to reconstruct past CO variations over the last 200 years. Consistent results in terms of atmospheric concentrations are obtained. We find that CO concentration started to increase over Greenland around 1850 and that, by contrast, CO levels at high southern latitudes remained fairly constant between 1860 and 1916. Based on available data on past CO sources, a scenario is proposed for the CO increase observed in Greenland. In addition, our Greenland CO results suggest that simulations of preindustrial CO distribution could have underestimated CO concentrations mainly in the northern hemisphere leading to an overestimate of the change since preindustrial times, and to an underestimate of the past interhemispheric gradient of carbon monoxide. ¿ American Geophysical Union 1996