Portable monitors were used to measure time averaged personal exposures (10-30 min) to carbon monoxide. Data were collected from January through March 1981 in four cities where carbon monoxide ambient levels in excess of National Ambient Air Quality Standards have been reported: Stamford, CT; Los Angeles, CA; Phoenix, AZ; and Denver, CO. In each city, personal exposures were measured in three typical microenvironment types: 'indoor,' 'commuting,' and 'residential driving.' These measurements were made in the vicinities of fixed monitoring stations that recorded ambient levels of carbon monoxide. This study design permitted the subsequent comparison of simultaneous personal exposures and ambient concentrations.