Mouse embryos (strain NMRI) of days 9 + 2h, 9 + 12h, 10 + 2h, 10 + 12h, 11 + 2h, 12 + 2h, 13 + 2h, and 14 + 2h were fixed (i) in 3% glutaraldehyde +3% paraformaldehyde, (ii) in 1% glutaraldehyde +1% tannic acid, or (iii) in 1% glutaraldehyde +1.5% ruthenium red. The electronmicroscopic picture of the basement membrane (BM) changed depending on the fixative used. Addition of tannic acid led to a higher staining intensity of glycoproteins, whereas after ruthenium red proteoglycans were more heavily stained. The BM around the neural tube and around the epithelial tubules in the lung anlage were investigated electron microscopically. After fusion, the BM in the dorsal regions of the neural tube is missing; on days 9–10 it is, however, reformed. Between days 11 and 13 wide gaps in the BM of the lung anlage occur at the growth buds of the epithelial tubes, which are bridged on late day 13. In the basal parts of these two epithelial types membrane-bordered granules of different density occur singly or in groups. It is postulated that these structures contain BM-material and represent secretion granules. After secretion, BM-material is first bound to the cell membrane. This process is important for the initiation of the formation of the BM. Further growth, however, proceeds via lateral aggregation (self-assembly). Thus, intercellular gaps are bridged independent of the cell membrane. The process of lateral aggregation may also explain deviations from the normal course of the BM.