The islets of Langerhans in normal mice were analyzed and quantitated morphologically after selective in situ perfusion staining with dithizone of the isolated pancreas. Total islet area and number of islets were determined in a texture-analyzing system on projected images of each islet. Islets varied considerably in size and in shape and did not conform with ideal spheroids or ellipsoids. In order to derive methods for estimation of the total number and total volume of islets, a distributional study of the areas was carried out. The size distribution of the islets of Langerhans was not of a known simple type, and was fitted best with a (power)-generalized inverse Gaussian distribution.