The effect of the introduction of low-emission vehicles on potential air quality improvement in the Los Angeles area was predicted using a three-dimensional airshed simulation model. The simulations were based on ozone concentration estimates made on the basis of data released by the California Air Resources Board concerning projected quantities of emissions from various sources in 2010. Analyses were made of three scenarios. One assumed that LEV, ULEV and ZEV regulations were enforced as planned, a second assumed that these planned regulations were modified; and a third assumed that emission levels from various sources were reduced in line with the goals of the Air Quality Management Plan formulated by the South Coast Air Quality Management District. The results indicate that: (1)the quantity of reactive organic gases (ROG) emitted by vehicles would be cut in half by 2010 by the replacement of older model year vehicles even without the enforcement of the LEV regulations; the LEV regulations would lower the ROG level by another 50%, with the result that the contribution of vehicular emissions to the atmospheric ozone concentration would be reduced to approximately 25%, as compared with 50% in 1987; (2)scaling back the ULEV and ZEV regulations to the LEV level would not seriously affect the projected reduction in the ozone concentration, as the rate of reduction would decrease by less than 1%. The results also show that lowering the ROG level of vehicular emissions has a large effect on reducing the ozone concentration. In contrast, a reduction in NOx emissions has a smaller effect, and some of the data suggest that the ozone concentration might conversely increase under certain conditions. In 2010, when the contribution of vehicular emissions to the ozone level will be smaller, the simulation results suggest that a further reduction in the ozone concentration will most likely require a substantial decrease in the ROG level from non-vehicular emission sources.