A morphological study of the bovine pancreas from fetus to adult documents the presence of two distinct types of pancreatic islets: large islets, 100 to 1600 micron in diameter, enmeshed in interlobular connective tissue; small islets, 25-200 micron in diameter, enmeshed in exocrine tissue. Large islets consisting primarily of well granulated B cells, decrease in relative volume with increasing age and in the adult are seldom seen. The overall relative volume of endocrine tissue is age dependent and ranges from 30% in the sixth month fetus to 10% in the neonate and 5% in the adult. Small islets contain B cells that increase their cytoplasmic secretory granularity with increasing fetal age, significantly degranulate just prior to birth and subsequently regranulate several weeks after birth. Beta cells of the small islets are uniquely characterized by junctional complexes in close association with large numbers of maculae adherentes (desmosomes). Using lanthanum-hydroxide and freeze-fracture techniques the junctional complexes are shown to consist of macula occludens (focal tight junctions) enclosing nexuses (gap junctions). The two types of islet differ in distribution, times of growth and times of B-cell granularity and may be indicative of functional differences yet to be elucidated.