Tetrandrine (TET, 100 mg/kg) injected intraperitoneally could prevent rats from diabetes induced by alloxan. After 48 h of injection of alloxan, the blood sugar of the preventive group decreased from the control value of 25.46 +/- 1.21 mmol/L to 7.63 +/- 0.44 mmol/L (P < 0.001) while the serum insulin level increased to 11.33 +/- 1.97 microU/ml and the plasma glucagon concentration to 66.85 +/- 5.07 pg/ml respectively from the control group's value of 7.13 +/- 0.45 microU/ml and 90.35 +/- 8.33 pg/ml. In glucose tolerance test, TET 100 mg/kg showed that the abnormal glucose tolerance induced by alloxan could be improved. The blood sugar area under the glucose tolerance curve of the preventive group decreased from the control groups level of 113.28 +/- 5.02 mmol.h/L to 29.45 +/- 1.63 mmol.h/L (P < 0.001). Immunohistochemical observation of islet beta cells confirmed that TET could markedly prevent beta cells from injuries induced by alloxan and there was no obvious change in the appearance of islet beta cells in the preventive group. The above results suggested that TET could protect pancreatic islet beta cells from injuries caused by Alloxan.