Abstract. Chenopodium murale plants, induced to flower by 5 days of continuous light, produced 43% more ethylene than vegetative plants kept under short days (16 h darkness, 8 h light) . The 1-aminocyclopropane-1-carboxylic acid (ACC)-induced ethylene production, using saturating ACC concentration (10 mol -m -3 ) was also 55% higher in induced plants . Their ACC and N-malonyl-ACC (MACC) levels were also higher, the former increasing by 56% in both shoots and roots, the latter by 288% and 108% in shoots and roots, respectively. Administration of labeled [2,3-14 C)ACC produced a very similar relative content of ACC and MACC in both treatments . The only process influenced by flower induction was ACC conversion to ethylene . Induced plants converted 66% more ACC than the vegetative ones . The effects of photoperiod on ethylene formation and metabolism in a long-day plant (LDP) C . murale and a short-day plant (SDP) C. rubrum are compared . Ethylene formation seems to be under photoperiodic control in both species, but its role in flower induction remains obscure .Photoperiodic flower induction in the obligatory SDP Chenopodium rubrum was recently shown to be accompanied by a decrease in ethylene formation (Machairkovd et al . 1988). The contents of ACC and MACC did not change during the induction . Experiments using labeled ACC established that the reaction influenced by flower induction manifested itself as ACC to ethylene Abbreviations: ACC, 1-aminocyclopropane-l-carboxylic acid ; MACC, N-l-malonylaminocyclopropane-l-carboxylic acid ; SDP, short-day plant ; LDP, long-day plant .