Abstract: Changes of endogenous hormone contents in terminal buds of poplar 1-69 from autumn to spring were determined by bioassay and HPLC. It was found that the activity of growth-inhibiting substances was increased in autumn and decreased in winter and spring. Content of ABA was much lower (65μg/kg F. W.) in April 8 than in Oct. 15(972μg/kg F. W.). Obvious increase in GA activity was observed during the release of bud dormancy. From Oct. 15 and thereafter, GA3 content increased gradually and reached its maximum (342μg/kg F. W.) by Apr. 8. Content of B-ABA and B-GA3 was negatively correlated to F-ABA and F-GA3 respectively.The experiment also showed that exogenous GA3 and GA4+7 were effectively in breaking the bud dormancy of poplar twigs in greenhouse. On the other hand, treatment with ABA delayed the ending of bud dormancy. However, exogenous hormones treatment did not affect the bud dormancy of poplar twigs if they were collected on Oct. 12. But dormancy of poplar seems to be regulated by the interaction between growth-promoting and growth-inhibiting substances.