In order to assess the chronic impact of tropospheric ozone on vegetation in the Seoul metropolitan area, it is necessary to quantify ozone exposure. Two ozone indices commonly used to relate ozone exposure to injury of vegetation were calculated. SUM06(SUM of hourly concentrations at or above 0.06 ppm) and AOT40(Accumulated exposure Over a Threshold of 40 ppb) which are widely used as ozone indices in the US and Europe were calculated based on hourly ozone concentrations in 5 areas of Seoul and 5 cities of Kyunggido during 1990~1997. Most SUM06 levels were 1~5ppm.hr, however several areas in Northern and Eastern Seoul reached about 5~7 ppm.hr in 1996~1997. AOT40 values were as high as 17~24 ppm.hr. Although measured SUM06 levels would not be expected to significantly impact vegetation, the overall ozone index, as well as annual average, 95th, and 99th percentile have increased continuously over the last 8 years. Often, ozone concentrations are lower in cities where there is a significant NOx concentration, than in outlying rural agricultural areas where NOx scrubbing is not as important. Concentrations greater than 40 ppb, which can cause chronic ozone toxicity to vegetation, were found mostly in the summer and constitutued about 5~15% of total hourly ozone cocentrations.