Most, if not all, endocrine cells seem capable of synthesizing and storing more than one hormone. Such cellular colocalization of hormones can be due either to the presence of two or more specific granules within the cells or to colocalization of the hormones within a single granule. The present study was performed to clarify the subcellular localization of insulin, glucagon, somatostatin, and pancreatic polypeptide within the endocrine cells of the human and porcine pancreas during fetal development, with special reference to possible colocalization of the hormones. The tissue specimens were processed for ultrastructural cytochemistry using Lowicryl as embedding medium. An immunogold labeling technique was used with two parallel, but not interacting, antibody chains. Sections from each specimen were double labeled in different combinations giving a complete covering of the four major islet hormones. During fetal life (50-90 days prenatally in porcine pancreas, 14 weeks gestation in the human pancreas) several hormones were demonstrated, not only in the same endocrine cells, but also in the same secretory granules (polyhormonal granules). Costorage of insulin, glucagon, somatostatin, and pancreatic polypeptide was demonstrated in granules in pancreatic endocrine fetal cells. At an early fetal stage, the endocrine cells contained either dense, round granules or pale, heteromorphous granules. With increasing age and maturation of the endocrine cells, structural differentiation of the secretory granules was found to be associated with a gradual disappearance of the polyhormonal granules. The first genuine monohormonal cell to appear in the porcine fetus was the pancreatic polypeptide cell (at 70 days gestation); it was followed by the somatostatin-producing endocrine cell. Mature insulin- and glucagon-producing cells were only demonstrated after birth. Thus, in the adult pancreatic endocrine cells, each specific endocrine cell type produced only one of the four classical hormones. The present investigation demonstrated that the endocrine cells of the fetal, but not the adult, pancreas are able to synthesize all the major islet hormones, and that these peptides are costored in the same granule. The data obtained support the concept of a common precursor stem cell for pancreatic hormone-producing cells.