The cytodifferentiation of the human endolymphatic sac was studied in the period between gestational weeks 8 and 20. This period is of particular interest since it covers the major part of the morphological and functional maturation of the human inner ear. The studied time, i.e. 8-20 weeks of gestation, may be divided into three periods. Before week 10, between weeks 11 and 15 and weeks 15-20. While the endolymphatic sac appears as a simple slit-like appendage in the first period, until week 10 with cells of uniform size and shape, a beginning cytodifferentiation occurs in the second period between weeks 10 and 15. Thus the cells attain a more mature shape with different stainability, i.e. light- and dark-staining cells, as well as open lateral intercellular spaces indicating some functional maturity. In the third period, after week 15, the endolymphatic sac more or less seems mature with a rugose appearance in its proximal portion and a more even, slit-like appearance in the distal portion. The cells are differentiated with morphological signs of functional maturation. It may be concluded that the development of the endolymphatic sac roughly follows that of the cochlea which is regarded to be functionally mature in the beginning of the second half of pregnancy.