It was revealed by light microscopic experiments that in Brunner's glands and the epithelium of the human and mammalian duodenum, cells were present, with basally lying granulations. These granules could be identified at the ultrastructural level as accumulations of pleomorphe electron-dense granules (‘argentaffin’, ‘chromaffin’ and ‘osmiophilic’ granules). Depending on the light and electron microscopic staining techniques used, these cells were classified as ‘mucous’, ‘serous’, or ‘endocrine’ (‘enteroendocrine’, ‘paracrine’ and ‘enterochromaffin’) cells. In the present study, the following ultrastructural morphological criteria of Brunner's gland cells were correlated: size, shape and electron density of their nuclei and nucleoli with morphological changes of their cytoplasmic and intranuclear organelles, which serve the synthesis of mucopeptides. This proved that the synthesis of mucopeptides occurred in 3 phases. The first phase served the intranuclear synthesis of organelles, which latterly were used for the cytoplasmic mucopeptide synthesis. This phase was characterized by the appearance of an electron-lucent and euchromatic nucleus, which also contained an euchromatic and electron-lucent nucleolus, and a thin electron-dense nucleus membrane (euchromatic phase). Only a few freely lying ribosomes. membrane-bound ribosomes (rough endoplasmic reticulum, RER) and mitochondria were dispersed throughout the cytoplasm. Therefore, the cytoplasm appeared electron-lucent and nearly empty. The seccond phase was characterized by a round but heterochromatical nucleus containing also a heterochromatic nucleolus, and the appearance of receptosome- (endosome, endocytotic vesicle) and lysosome-like structures near to the cytoplasmic membrane of the basal cell portion, as well as in the basal and lateral perinuclear region (early heterochromatic phase). The receptosome-like structures were round-oval and contained a single electron-lucent inclusion vesicle, which had an empty space or an electron-opaque granule. Both inclusion structures were located eccentrically on the inner surface of the membrane of the receptosome-like structure. Here and there, it was possible to visualize great receptosome-like structures (multivesicular bodies) containing numerous membranous electron-lucent and electron-opaque inclusion vesicles. Electron-dense pleomorphe lysosome-like structures, bearing 1–4 electron-lucent inclusion vesicles with faint grains in their spaces, were also recognized in the basal cell portion, as well as in the basal and lateral perinuclear region. Strikingly, often receptosome-like structures containing electron-dense or electron-lucent materials were in close contact with lysosome-like structures in the perinuclear region, but rarely in the apical cell portion. Moreover, large inclusion-containing receptosome-like structures were in close association with Golgi apparati in these regions. In the apical cell portion only some receptosome- and lysosome-like structures were seen. With increasing heterochromasis of the nucleus and nucleolus, and with increasing dentating of the nucleolus, the receptosome- and lysosome-like structures disappeared and smooth endoplasmic reticulum structures flowed from the nucleoplasm into the cytoplasm via open pores (diameter 60 mm) of the nucleus membrane. Ribosome-like structures (diameter 20 nm) then flowed intensely through these pores into the cytoplasm, expressing the beginning of the cytoplasmic synthesis of mucopeptides (late heterochromatic phase). In addition, an intense increase of the cytoplasmic electron density, produced by lamellar RER structures, were noted mainly in the basal and perinuclear cell region. Thereby, the nucleus was more and more dntated heterochromatically and more and more dislocated to the basal cell portion by large mucous vacuoles formed in the apical cell portion. Brunner's gland cells obtained, therefore, a goblet-like shape. After rupture of the apical cell membrane, the mucous-containing vacuoles flowed into the duodenum lumen. These findings might permit the conclusion that Brunner's gland cells do not contain ‘endocrine’ cells, and that the supposed ‘endocrine’ cells represent a functional intermediate phase of the mucopeptide synthesis which occurs within these cells. Therefore, they were so far interpreted as an independent cell type because they were observed only under non-dynamic snapshot-like circumstances.