The mechanism of functional disorder caused by treatments with MH, KCN, acetaldehyde and/or by enclosing in desiccator was studied with onion bulbs (Yellow Danvers Flat) during the storage period. The experimental results are outlined as follows; (1) In the tissues received fungal infection and functional disorder, the accumulation of ethyl alcohol and acetaldehyde were found, and the acetaldehyde reaction with SCHIFF's agent was strongly positive in the bulbs injured severely. (2) The total sugar content was decreasing with the advance of disorder, and the decrease of sugar content was always lower in different degrees, when sudden disorder was induced by artificial treatments above mentioned. (3) The total content of ascorbic acid was little decreasing during the development of functional disorder, whereas l-ascorbic acid was oxidized to dehydroascorbic acid. (4) With increasing degree of disorder, the inceased O2 uptake and CO2 output were evident in the disk part, both reaching to the peak and then rapidly decreasing to an extent much lower than the normal material. (5) The variations of the activity of catalase and peroxidase in the disk part were showing the same tendency as that of the gas exchange pointed out in (4). (6) The acetaldehyde of certain concentration inhibited the dehydrogenase activity in disk part, in vitro as well as in vivo. (7) Judging from the results given above, it appears that the onions stored under such environ-mental condition in which the dehydrogenase activity in TCA cycle was inhibited, show an accumulation of acetaldehyde in their tissues and this must be an immediate cause of functional disorder of onions in storage.