Nineteen oocytes aspirated from mature Graafian follicles of normally cycling cows in standing oestrus were studied. Oocytes were fixed in phosphate-buffered glutaraldehyde followed by exposure to osmium tetroxide and embedding in Epon 812. Oocytes could be classified into four types based on characteristics of nuclear components and distribution of organelles and inclusions. These types represented late maturational changes occurring shortly before ovulation. The close association of granulosa-cell processes in Type-I maturing follicular oocytes showed progressive signs of deterioration in other oocyte types. In all oocytes, numerous mitochondria with 'hood-like' appendages were observed. Single cisternae of endoplasmic reticulum were closely associated with the surface of mitochondria and the cavity formed by the 'hood-like' appendage. Endoplasmic reticulum was also associated with lipid droplets and was observed to be continuous with the outer leaflet of the nuclear envelope. Infrequently, a more classical granular endoplasmic reticulum was apparent. The cytoplasmic matrix was moderately dense and contained scattered ribosomes and polysomes. Abundant vesicles containing small PAS-positive granules were evenly distributed throughout the ooplasm. Golgi dictyosomes, annulate lamellae, cortical granules and two types of nucleoli characterized these oocytes.