The promotive effect of AgNO3 and aminoethoxyvinylglycine (AVG) on in vitro shoot regeneration from cotyledons of Brasska campestris ssp. peklnensls in relation to endogenous 1-aminocyclopropane-1-carboxylic acid (ACC) synthase, ACC, and ethylene production was investigated. AgNO3 enhanced ACC synthase activity and ACC accumulation, which reached a maximum after 3 to 7 days of culture. ACC accumulation was concomitant with increased emanation of ethylene which peaked after 14 days. In contrast, AVG was inhibitory to endogenous ACC synthase activity and reduced ACC and ethylene production. The promotive effect of AVG on shoot regeneration was reversed by 2-chloroethylphosphonic acid at 50 micromolar or higher concentrations, whereas explants grown on AgNO3 medium were less affected by 2-chloroethylphosphonic acid. The distinctive effect of AgNO3 and AVG on endogenous ACC synthase, ACC, and ethylene production and its possible mechanisms are discussed.Plants usually exhibit enhanced ethylene production when wounded or under environmental stresses or pathogen attack (31). Ethylene biosynthesis in plants occurs via SAM' and ACC intermediates using methionine as a precursor (31), but the molecular mechanism of its action is not clear. It has been known for some time that cultured plant cells evolve ethylene (1 1 Cotyledonary explants excised from 3-d-old aseptically germinated seedlings of Brassica campestris ssp. pekinensis cv Shantung and cv Wong Bok were used. Because our preliminary investigations showed that explants cultured in 50-mL Erlenmyer flasks and 100-x 25-mm Petri dishes exhibited the similar capacity of shoot regeneration, both culture containers were used in this study. Explants were cultured either in a flask containing 25 mL medium or in a Petri dish containing 30 mL medium consisting of all constituents of Murashige and Skoog's medium (16)