INSULIN secretion was traditionally measured with biochemical and immunological methods such as enzyme linked immunosorbant assay and radio-immunoassay.However,these methods can only tell the amount of insulin secreted; they give no information about the secretion process or mechanism of exocytosis.In recent years,an imaging technique known as total internal reflection fluorescence (TIRF) microscopy has been employed to study insulin secretion.1-4 This imaging technique can explore events taking place near or on live cell membrane,such as secretory granule movement,exocytosis,vesicle content release,and membrane fusion.5-10 In the present paper,we applied TIRF microscopy to the observation of insulin exocytosis by the pancreatic β cell line Ins-1.