Objectives: To study the effects of Parthenium hysterophorus L. flower on serum glucose level in normal and alloxan induced diabetic rats. Materials and Methods: Albino rats were divided into six groups of six animals each, three groups of normal animals receiving different treatments consisting of vehicle, aqueous extract of Parthenium hysterophorus L. flower (100 mg/kg) and the standard antidiabetic drug, glibenclamide (0.5 mg/kg). The same treatment was given to the other three groups comprising alloxan induced diabetic animals. Fasting blood glucose level was estimated using the glucose oxidase method in normal and alloxan induced diabetic rats, before and 2 h after the administration of drugs. Results: Parthenium hysterophorus L. showed significant reduction in blood glucose level in the diabetic (P<0.01) rats. However, the reduction in blood glucose level with aqueous extract was less than with the standard drug glibenclamide. The extract showed less hypoglycemic effect in fasted normal rats, (P<0.05). Conclusion: The study reveals that the active fraction of Parthenium hysterophorus L. flower extract is very promising for developing standardized phytomedicine for diabetes mellitus.