The effect of polyamines and methylglyoxal bis(guanylhydrazone)(NGBG) on the activity of diamine oxidase was studied in soybean (Glycine max) seedlings. 10^-2 M of putrescine, spermidine, and spermine inhibited diamine oxidase activity, whereas 10^-6 M putrescine increased enzyme activity. These results suggest that diamine oxidase can be induced by a specific subastrate, putrescine. The content of putrescine was increased in response to the increase in concentratioin of MGBG. In vitro, 40% of the diamine oxidase activity was inhibited by 10^-3 M MGBG. In vivo, the diamine oxidase activity was increased by a low concentrration of MGBG. It was suggested that MGBG inhibited the formation of spermidine and that the accumulated putrescine induced diamine oxidase, whereas the diamine oxidase acitivity was inhibited by a high concentration of MGBG. It is suggested that a high cocentration of MGBG increases the putrescine content by inhibiting diamine oxidase activity which is responsible for putrescine degradation.