Carbon monoxide concentrations were measured at ground level (1 m) near heavy traffic streets in downtown Santiago de Chile in periods of low (November and December), intermediate (April) and high (May) ambient concentrations. Also, measurements were carried out at several heights (from 1 to 127 m) in Santiago's main street during winter time. Measurements carried out at ground level show maximum values during the morning rush hour, with values considerably higher than those reported by the urban air quality network, particularly in summer time. However, the measured values are below air quality standards. Vertical CO profiles were measured in a tower located in the center of downtown. Below 40 m (average altitude of neighboring buildings), the profiles do not show a consistent vertical gradient, with CO concentrations increasing or decreasing with height, regardless of atmospheric stability. In this low altitude range, the observed vertical profiles are poorly predicted by a street canyon model, and the measured concentrations can not be described by a simple exponential decay. At higher altitudes (40 and 127 m) a negative gradient in CO concentrations is observed, both for stable and unstable atmospheric conditions. The values of CO measured at 127 m are relatively well described by an Eulerian dispersion model running with current CO emission inventories for Santiago, although this model tends to predict stepper CO gradients than the observed ones.