The antidiabetic potentials of methanol extract (ME) of aerial parts of Phyllanthus niruri L (Euphorbiaceae) was evaluated in normal and alloxan diabetic rats. The results showed that ME significantly (P<0.05) reduced fasting blood sugar in a dose-related manner and suppressed the postprandial rise in blood glucose after a heavy glucose meal in normoglycaemic rats. Chronic oral administration of ME caused a significant (P<0.05) dose-related reduction in blood glucose levels as well as total cholesterol and triglycerides levels in diabetic and normoglycaemic rats. Sub-chronic toxicity study showed that ME-treated rats had significant (P<0.05) reductions in haemoglobin (Hb) levels, red blood cell (RBC) and white blood cell (WBC) counts followed by a gradual rise which did not, however, attain basal levels; however, there was a progressive rise in the WBC of ME-treated diabetic rats. Also ME-treated and control rats had increases in weight throughout the study. Histological studies showed that ME-treated diabetic rats had the tissue architecture of their pancreas restored as against the control groups where there was evidence of necrosis.