Medicinal plants have been a source of succour in the control of many diseases in developing countries. The toxic effects of an aqueous extract of Senna siamea were studied in 20 male albino ratsâ€™ liver and kidney over a period of 7 days. The rats were divided into four groups of five rats per group. Those in Group A served as controls while the rats in Groups B, C and D were dosed with 100 mg/kg of the extract for different number of days. The results of this study reveals a drastic reduction (P<0.05) in the activities of Alkaline Phosphatase (ALP), Aspartate Transaminase (AST) and Alanine Transaminase (ALT) in the liver with a corresponding increase in the serum levels, indicating a mild liver damage which returned to normal when the administration of the extract was stopped. There was a significant increase in the activities (P<0.05) of AST and ALT in the kidney and the serum which might be caused by activation of enzymes synthesis in renal cells. However the reduction of ALP in the kidney and increase in the serum indicated tissue damage. The test results also revealed hypoglycaemic properties of the extract. The results obtained shows that prolonged usage of the aqueous extract of Senna siamea may lead to cell destruction.