The pars distalis of 1 girl and of 3 sexually mature women was studied. The glands were dissected out and fixed within a period of 30 min following death; in the aldehyde fixative, each gland was systematically divided into ten pieces. Before fixation in osmium tetroxide, each piece was again divided into several blocks. At least 60 blocks were obtained from each gland. Semi-thin sections obtained from all the blocks were stained and studied under the light microscope. Ultrathin sections were obtained from selected blocks. Under the light microscope, 6 types of secretory cells and 2 types of non-secretory cells were characterized. At the ultrastructural level, 7 types of granulated cells and 2 types of non-granulated cells were distinguished. The range of the granule size for each type of secretory cell was determined by applying a mathematical model and a computer program that corrected the size distribution curve in such a way that only the diameter of the equatorial sections of the granules, that is the real granule sizes, was recorded. By comparing the observations made in the ultrastructural study with those performed in adjacent semi-thin sections it was possible to correlate the cell types characterized ultra-structurally with those distinguished under the light microscope. In addition, the study of the semi-thin sections made it possible to quantitatively analyze the different cell types, while the ultrastructural characteristics suggested the probable functional role of each of these cell types.