Objective:To elucidate the adaptive changes on the morphology and proliferation of pancreatic islet during pregnancy in rats. Methods:Rat pancreatic tissues of each group were double-stained by 5-bromodeoxyuridine(BrdU) and anti-insulin antibody by immunohistochemistry technique to measure the changes of islet morphology and quantity during pregnancy at day 10.5,12.5, 14.5,18.5 and 20.5. Results:Compared with the normal control group, the average photodensity of insulin-expression positive zone, insulin relative concentration(IRC), insulin-expression positive cell density(PCD) and proliferation index(PI)of pancreatic islet were all significantly increased on day 12.5, rose continuously to a peak at day 14.5 of pregnancy, and then gradually returned to levels as control. In addition, areas of insulin-expression positive cells were also much greater during pregnancy during day 14.5 to day 20.5 than in non-pregnant rats. Conclusion:Both pancreatic islet -茁 cell proliferation and insulin-expression concentration became more significantly increased than sex-matched virgin control rats from day 12.5, peaked at day 14.5 of pregnancy,and lasted to late pregnancy(day 20.5).