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Responses to oligogalacturonic acid (OGA) were determined in transgenic Arabidopsis thaliana seedlings expressing the calcium reporter protein aequorin. OGA stimulated a rapid, substantial and transient increase in the concentration of cytosolic calcium ([Ca 2+ ] cyt ) that peaked after ca. 15 s. This increase was dose-dependent, saturating at ca. 50 µg Gal equiv/ml of OGA. OGA also stimulated a rapid generation of H 2 O 2 . A small, rapid increase in H 2 O 2 content was followed by a much larger oxidative burst, with H 2 O 2 content peaking after ca. 60 min and declining thereafter. Induction of the oxidative burst by OGA was also dose-dependent, with a maximum response again being achieved at ca. 50 µg Gal equiv/mL. Inhibitors of calcium fluxes inhibited both increases in [Ca 2+ ] cyt and [H 2 O 2 ], whereas inhibitors of NADPH oxidase blocked only the oxidative burst. OGA increased strongly the expression of the defence-related genes CHS, GST, PAL and PR-1. This induction was suppressed by inhibitors of calcium flux or NADPH oxidase, indicating that increases in both cytosolic calcium and H 2 O 2 are required for OGA-induced gene expression.