Estradiol (E 2 ) was measured in spring and winter tissues in several populations of dioecious Populus tremuloides Michx. and in four trees with perfect flowers. A correlation between the concentration of E 2 and sex expression was observed. In catkins, E 2 concentrations increased before anthesis, peaked during flowering, and decreased as flowers matured. The increases were concurrent with sporogenesis and development of gametophytes. Statistically significant seasonal variation in E 2 concentration was also evident. Dormant winter tissues showed significantly lower E 2 concentrations than spring tissues. Branches exposed to relatively more light had significantly higher E 2 content in both spring and winter. Seasonal variation in E 2 content was also observed in trees of the same sex, among branches of the same tree, and among different organs.