Development of the mouse choroid plexus was studied by semithin light microscopy, transmission electron microscopy and scanning electron microscopy. The choroid plexus is first observed as a bilateral ridge at 11 days postconception. The major morphological development appears to occur between 11 and 14 days postconception. By 14 days both dark and light choroid plexus epithelial cells are present. The percentage of dark cells appears constant from 14 days postconception up to 3 months postnatum. Metachromatically staining glycogen masses are present in the choroidal epithelium from 13 days postconception until 5 days postnatum, after which time glcogen granules are sparsely scattered throughout the cytoplasm. A few fine microvilli are present at 11 days postconception and these increase in number and become much more bulbous by 13 days. In contrast to the light choroid plexus epithelial cells, the dark cells have fine narrow microvilli. The possible significance of the two types of choroid plexus cells is discussed.