Fresh autopsy materials of the kidneys from five healthy executed men and surgical materials from ten patients were fixed with LEVI's solution, REGAUD's solution, ZENKER-formol and 10% formol. Paraffine sections were stained with HEIDENHAIN's iron-hematoxylin, azan, KULL's method and periodic acid-SCHIFF (PAS) reaction. In these preparations epithelial cells of the proximal convolution in renal tubules were studied cytologically.In both, pars convoluta and recta of the proximal convolution, the free surface of epithelial cells are covered by the brush border which consists of multitude of minute hair-like processes and shows the same striated structure as in the cuticula of intestinal epithelium. Each processes have, beneath the free surface of the cells, minute basal corpuscles which in optical section appear as a dark stained line. The brush border reacts generally positively upon periodic acid-SCHIFF reaction.Epithelial cells of the proximal convolution are generally rich in mitochondria; in the basal parts of cells up to the paranuclear regions a majority of rod-shaped and filamentous mitochondria are arranged densely and perpendicularly to the basement membrane, making the basal striation. Relatively small number of rod-shaped mitochondria are also scattered irregularly in the supranuclear parts, where, besides the mitochondria, variable number of granules and vacuoles are found in normal condition. Granules derived, probably, from the mitochondria are deeply stained with iron-hematoxylin, and react, however, negatively to PAS. Vacuoles seem to be produced from liquefying granules. These phenomena found in supranuclear parts of epithelial cells of the proximal convolution suggest that these cells have secretory activity.The free surface of dark epithelial cells of the proximal convolution often send into the lumen long cytoplasmic protrusions of paipllar shape which appear quite clear and contain various number of stainable granules and vacuoles. The brush border found on the surface of the basal part, becomes, however, thinner towards the apical part and disappears completely. The apical part of these protrusions are constricted and thrown off as droplets or fragments into the lumen or the cytoplasmic content flows into the lumen through the broken tips of the protrusions. In these manners the granules and vacuoles are released from the cells. These phenomena indicate apocrine secretion of epithelial cells.It is generally accepted that the epithelium of the proximal convolution has also absorption activity. In the epithelium of this part a number of large, swollen epithelial cells are found here and there bulging as clear eminences into the lumen. On the free surface of these cells the brush border remains intact which is generally thought to participate in absorption. These cells, therefore, probably represent those engaged in absorptive function.The free surface of the epithelium of human proximal convolution is always uneven and intensely rugged, so that the lumen is irregular in shape and narrow. These facts chiefly depends on the morphological changes of epithelial cells induced by functional activities as absorption and apocrine secretion.GOLGI apparatus in epithelial cells of human proximal convolution demonstrated by KOLATCHEV's osmium-impregnation method consists of many short curved threads (apparatus elements) which are arranged on a line running transversely at the level of the nucleus without making net-like structure. In parts containing the nucleus they are found in the supranuclear or paranuclear region. They occur, however, rarely in the infranuclear region, too. No remarkable variations of the structure and location of GOLGI apparatus are found in this study.The chief morphological difference of pars recta to pars convoluta is in its poor development of the basal striation found in the epithelial cells.