Ethylene induced an increase in the level of 1-aminocyclopropane-1-carboxylate (ACC) oxidase mRNA but suppressed the expression of ACC synthase transcript in mung bean hypocotyls. Induction of ACC oxidase transcript by ethylene was saturated at a low concentration (1–3 µl l−1) and detectable within 1 h after ethylene treatment; 2,5-norbornadiene, an inhibitor of ethylene action, significantly reduced the basal level of ACC oxidase transcript present in intact mung bean hypocotyls. Treatment with the protein synthesis inhibitor cycloheximide completely inhibited the ethylene-induced accumulation of ACC oxidase transcript, indicating that de novo protein synthesis is necessary for expression of the ethylene-inducible ACC oxidase gene. ABA decreased the ethylene-induced accumulation of ACC oxidase mRNA, but restored the ethylene-suppressed level of ACC synthase (VR-ACS1) transcript. The protein kinase inhibitor staurosporine effectively prevented ethylene-induced ACC oxidase gene expression, whereas it substantially recovered the ethylene-suppressed transcript level of ACC synthase, suggesting that protein phosphorylation plays a role in the induction of ACC oxidase and the suppression of ACC synthase by ethylene in mung bean hypocotyls. Okadaic acid, a potent inhibitor of protein phosphatase, did not affect the expression of ACC oxidase. However, addition of okadaic acid along with ethylene effectively blocked the ethylene-induced accumulation of ACC oxidase transcript, while there was an increase in the level of ethylene-suppressed ACC synthase transcript. These results indicate that protein dephosphorylation in addition to phosphorylation is necessary for the ethylene signaling that regulates the expression of these genes.