The author studied the ultrastructure of the cumulus oophorus in three groups of Graafian follicles divided according to the degree of maturity of the oocyte: follicles containing an oocyte with a germinal vesicle (group I), follicles containing a metaphase I oocyte (group II), and follicles containing a metaphase II oocyte (group III). Loosening of contacts between the corona radiata cells and the oocyte, as well as intercellular contacts of the cumulus oophorus cells, was observed after the resumption of meiosis and the deposition of finely granular material in the intercellular spaces (expansion and mucification). The cytoplasm of all the cells contained organelles typical of protein synthesis (granular endoplasmic reticulum, a large number of free ribosomes, a Golgi apparatus). There was practically no smooth endoplasmic reticulum. The inner mitochondrial membranes had processes in the form of cristae. Lipid droplets were present in small numbers. The morphology and number of the cytoplasmic organelles remained similar during the whole of the given period, with the exception of the lipid droplets, the number of which was slightly higher in group III. No morphological signs indicative of steroid hormone synthesis were found. During the whole of the preovulation period, the character of cellular proteosynthetic activity remained the same.