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Tomato (Lycopsicon esculentum Mill.) plants grown in a greenhouse were irradiated with two different levels of UV-B, namely 8.82 (T1) and 12.6 kJ/m2 per day (T2). Ethylene production, 1-aminocyclopropane-1carboxylate (ACC) content, 1-(malonylamino) cyclopvopane-1-carboxylic acid (MACC) content, gene expression of ACC synthase (EC 4.4.1.14), and ACC oxidase activity in tomato leaves were determined. The results indicated that ACC content, the activity of ACC synthase and ACC oxidase, and ethylene production increased continuously under low doses of UV-B radiation, whereas at high doses of radiation these parameters increased during the first 12 d and then started to decrease. The MACC content increased continuously over 18 d under both doses of UV-B irradiation. The changes in ACC content, ACC synthase activity,ACC oxidase activity, the transcriptional level of the ACC synthase gene, and ethylene production were consistent with each other, suggesting that ACC synthase was the key enzyme in ethylene biosynthesis and that ethylene production in tomato leaf tissues under UV-B radiation could be regulated by the expression of the ACC synthase gene. The results also indicate that the change in ethylene metabolism may be an adaptive mechanism to enhanced UV-B radiation.