Obesity is an epidemic in most developed countries and novel therapeutic approaches are needed. In the course of a screening project of medicinal plants used by the Eastern James Bay Cree of Canada and having potential for the treatment of diabetes, we have identified several products that inhibit adipogenesis, suggesting potential antiobesity activities. The inhibitory activity of two of these, the extract of the inner bark of the deciduous trees Alnus incana ssp. rugosa (Speckled Alder) and Populus balsamifera L. (Balsam Poplar), was analyzed using the 3T3-L1 cell model of adipogenesis. Intracellular triglyceride accumulation, pre-adipocyte proliferation, and PPAR- γ activity were measured. Alnus incana extracts acted early in the differentiation process but did not affect clonal expansion of pre-adipocytes nor the morphological transformation from fibroblast-like to rounded fat-laden cells. Alnus incana extracts were found to act as partial agonists toward PPAR- γ activity. In contrast, Populus balsamifera extracts completely abrogated adipogenesis, severely limited clonal expansion of pre-adipocytes and generally maintained cells in an undifferentiated fibroblast-like morphology. Populus balsamifera extracts exerted antagonistic action against PPAR- γ activity. It is concluded that, through their actions on the adipocyte, these plant products may be useful for the treatment of obesity and related metabolic diseases.