We compared the physiological and molecular responses of two Arabidopsis accessions, Col-0 and Ws-2, to ozone (O(3)) exposure. Observation of visible injury as well as ion-leakage analysis demonstrated clear differences between the O(3)-tolerant accession Col and the O(3)-sensitive accession Ws. RNA-blot analysis showed that O(3)-induced increases in mRNA levels of several ethylene-inducible genes and a salicylic acid-inducible gene were substantially higher in Ws than in Col. The time-course of induction of various mRNA levels shows that the expression of ethylene-inducible genes was rapidly, and more strongly, induced by O(3) in Ws than in Col, suggesting that Ws exhibits higher ethylene-signaling. Both the level of mRNA for an O(3)-inducible 1-aminocyclopropane-1-carboxylate synthase and the level of ethylene generation after 3 h of O(3)-exposure were higher in Ws than in Col. O(3)-induced leaf damage was attenuated by pretreatment with ethylene biosynthesis- and signaling-inhibitors, indicating that ethylene signaling is required for O(3)-induced leaf injury in Ws. On the other hand, an ethylene-overproducing mutant of Col, eto1-1, displayed significantly increased O(3)-induced leaf injury compared to wild type plants. These results indicate that the difference in O(3) sensitivity is dependent on the difference in ethylene production rate between these two accessions. Finally, we investigated the relationship between the degree of leaf damage and the level of ethylene evolution in 20 different Arabidopsis accessions. Based on the result, the accessions were classified into four types. However, most of them showed significant correlation between the ethylene production level and the degree of leaf injury, suggesting that ethylene signaling is an important factor in the natural variety of O(3) sensitivity among Arabidopsis accessions.