Tobacco Nicotiana tabacum L. Is a host plant of Helicoverpa armigera (Hüibner), Helicoverpa assulta Guenée and Spodoptera litura (Fabricius) (Lepidoptera, Noctuidae).The difference in leaf nicotine response to the feeding by these three larvae and the mechanical simulation of their feeding was examined by HPLC. Results indicated that nicotine induction was suppressed by H. Armigera and H. Assulta larvae feeding or by simulated damage treated with their labial glands extracts. The production of nicotine was also suppressed by the glucose oxidase from Aspergillus niger when it was treated on mechanically wounded leaf area. On the contrary, the nicotine production was stimulated by S.litura larva feeding or by simulated damage treated with its labial gland extract. Heat denature can not counteract the stimulation effect of the S. Litura labial gland extracts to tobacco nicotine production. The glucose oxidase activity was detected in labial gland extracts of both H. Arrnigera and H.assulta, but the activity in H. Armigera was significantly higher than that in H. Assulta. No glucose oxidase activity was detected in labial gland extracts of S. Litura. It is shown that the glucose oxidase activity in labial glands of caterpillars plays an important role in the nicotine response to herbivory. The glucose oxidase was mainly contained in the labial gland of H. Armigera larva, and had the highest activity at pH 7.0. D-Glucose was the optimal substrate of the glucose oxidase. Labial gland glucose oxidase activities varied daily during larval development with high activities found when larvae were actively feeding.