A cross between transgenic tobacco (Nicotiana tabacum L.) plants which over‐expressed either γ‐glutamylcysteine synthetase (cpGSHI) or glutathione synthetase (cpGSHII) in their chloroplasts was used to compare the consequences of over‐expression of components of the glutathione synthetic pathway on tolerance to atmospheric O3 or paraquat. A high proportion (50%) of those progeny which carried the cpGSHII transgene alone showed tolerance to atmospheric O3 but not to paraquat. Progeny of an additional two, independent, self‐pollinated primary transgenic lines, which segregated in a Mendelian fashion for the presence of the cpGSHII transgene and therefore included both transformed and non‐transformed (recessive, wild‐type) plants, were also challenged by fumigation with O3. Again, in both cases, about 50% of the plants expressing the epGSHII transgene were found to be O3‐tolerant on the basis of reduced ethylene emissions and increased or unchanged total pigment concentrations. However, this tolerance was not due to specific changes in stomatal densities.