We immunohistochemically examined the distribution of glucokinase (GK) in the B-cells of pancreatic islets of normal rats. GK was stained punctately in the cytoplasm of B-cells when examined under the light microscope. By use of a double-immunostaining technique, most of the GK immunoreactivity was observed to be colocalized with insulin immunoreactivity. Electron microscopic examination by the immunogold method revealed that GK immunoreactivity was predominantly located within insulin-secretory granules of pancreatic B-cells.