Carbon monoxide (CO) exposures were measured inside a motor vehicle during 88 standardized drives on a major urban arterial highway, El Camino Real (traffic volume of 30,500-45,000 vehicles per day), over a 13-1/2 month period. For the 88 trips, the mean CO concentration was 9.8 ppm, with a standard deviation of 5.8 ppm. Of 9 covariates that were examined to explain the variability in the mean CO exposures observed on the 88 trips (ambient CO at 2 fixed stations, atmospheric stability, seasonal trend function, time of day, average surrounding vehicle count, trip duration, proportion of time stopped at lights, and instrument type), a fairly strong seasonal trend was found. A model consisting of only a single measure of traffic volume and a seasonal trend component had substantial predictive power; by contrast, the ambient CO levels, although partially correlated with average exposures, contributed comparatively little predictive power to the model. This study shows: (1) the mass balance equation can relate exterior CO concentrations as a function of time to interior CO concentrations, (2) CO exposures on urban arterial highways vary with season, (3) momentary CO experienced behind red lights vary with the intersection, and (4) an averaging time model can simulate exposures during short trips (20 minutes or less) on urban arterial highways.