Treatment of the ozone-sensitive tobacco cultivar Bel W3 with an ozone pulse (0.15 microliter/l, 5 h) markedly increased the mRNA level of basic beta-1,3-glucanase and to a lower degree that of basic chitinase. The increase of beta-1,3-glucanase mRNA level occurred within 1 h and showed a transient maximum. Seventeen hours after ozone treatment, the beta-1,3-glucanase mRNA level decreased to lower values. The increase of basic chitinase mRNA level was delayed and was less pronounced than that of beta-1,3-glucanase mRNA. Cultivar Bel B showed only a small increase of beta-1,3-glucanase mRNA level after the same ozone treatment, whereas its basic chitinase mRNA was more strongly induced. Prolonged ozone treatment for 2 days of tobacco Bel W3 led to a persistent level of beta-1,3-glucanase and basic chitinase mRNAs, as well as to an increase of acidic chitinase and 'pathogenesis-related' (PR) 1b mRNA levels. The results indicate that genes so far considered to code for PR proteins may also be involved in the plant response to oxidative stress.