Ozone transport and chemistry in the springtime in east Asia are studied by use of the STEM-II (Sulfur Transport Eulerian Model) regional-scale transport/chemistry model. Three-dimensional simulations are performed for the period May 1--15, 1987. This was a period of strong downward transport of ozone in east Asia, associated with traveling low-pressure systems. Elevated ozone levels were observed at high-altitude surface sites in Japan during this period. Model simulations both with and without photochemical processes are performed in order to assess the relative importance of the transport and chemical sources of tropospheric ozone. The model results are compared with measured values at a network of stations in Japan and are found to accurately capture most of the important observed features. Near-surface ozone levels are found to be strongly influenced both by continental outflow of precursors occurring behind the cold fronts as they move out over the Pacific Ocean and by the strong downward transport of ozone-rich air from the upper troposphere which occurs in association with these weather systems. ¿ 1998 American Geophysical Union