The postnatal growth of the mouse pancreas was studied using stereological methods. The measurements obtained included gland mass, total cell number, the number and frequency of cells in each morphological compartment and the nuclear and cytoplasmic volume of the acinar cells. Pancreatic mass increased significantly (>10,000 fold) in the first 70 days of life and this was accompanied by an increase of 6,841% in the total cell number. The number of acinar, centroacinar, ductal and stromal cells increased by 9,241%, 7,027%, 4,864% and 3,360%, respectively. During the same period, the mean acinar cell volume increased by only 146%. These results showed that growth of the mouse pancreas during postnatal development, occurred through intense proliferative activity of all cell types and by an increase in the size of individual cells, notably the acinar cells.