To investigate the relatinship between methyl jasmonate (MeJA) and ethylene in leaf senescence, we studied the effects of MeJA on ethylene production and ethylene biosynthetic enzyme activities in oar (Avena sativa L.) leaf segments incubated in darkness. MeJA promoted dark-induced senescence judged from the contents of chlorophyll and protein, and increased ethylene production 6 times of the control. MeJA also increased the activities of ethylene biosynthetic enzymes, 1-aminocyclopropane carboxylic acid (ACC) synthase and ACC oxidase as compared to control. In MeJA-treated leaf segments, ACC synthase activity reached its maximum level at 24 h of incubatin and ACC oxidase activity peaked at 6 h of incubation. Aminoethoxyvinylglycine (AVG) and Co2+, inhibitors of ACC synthase and ACC oxidase respectively, reduced MeJA-induced ethylene production. They also delayed leaf senescence that was promoted by the treatment of MeJA. From these results, we can suggest that MeJA increased the activities of ACC synthase and ACC oxidase, these increased activities lead to increase in ethylene production and this increased ethylene production might promote dark-in-duced leaf senescence.