The postnatal development of the guinea pig pancreas (Cavia porcellus) from 2 to 140 days old was studied stereologically by light microscopy. During this period, the pancreatic volume increased 805%. The compartmental volumes of the acini, intercalated ducts, excretory ducts, islets of Langerhans and stroma increased 756%, 1,372%, 1,591%, 3,393% and 793%, respectively, whereas the total external surfaces of the acini, intercalated ducts and excretory ducts increased 542%, 1,667% and 1,002%, respectively. This growth involved increases of 361%, 74%, 1,355% and 673% in the total number of acinar, centroacinar, intercalated duct and excretory duct cells, respectively. In the same period, the mean volume of acinar cells increased 210%. These results indicate that the growth of the acinar compartmental volume involves a significant proliferative activity of the acinar cells and an increase in the individual cell volume. Analysis of the surface-to-volume ratio during this growth showed that the acini increased significantly in size. On the other hand, the growth of the parenchymal structures was accompanied by a reduction on interacinar connective tissue spaces and by the final organization of the interlobar and interlobular septa.