The development of the branchial arch region was examined at 6- to 12-hour intervals in mice embryos at the age of 9.5–11.5 days after conception [middle of embryonal day (ED) 10 to middle of ED 12] and in rat embryos at the age of 10.5–12.5 days after conception (middle of ED 11 to middle of ED 13) using SEM, serial semithin sections and wax plate reconstructions. The 4th branchial groove is always separated from the corresponding pharyngeal pouch by a broad layer of mesenchyme. At no time did we find a membrana obturans in this region. However, during development the mesenchyme disappears between the branchial grooves and pharyngeal pouches of the first four arches, allowing external ectodermal epithelium to come into contact with the internal endodermal epithelium to form a membrana obturans. In its dorsal parts this membrane is formed by two layers of epithelium, in the ventral portion frequently it consists of only one layer of epithelium. It was not possible to determine whether the latter was derived from ectoderm or endoderm. Degeneration of cells in this ventral part of the membrane leads to openings which we consider to correspond to branchial clefts. The possibility of an artefactual genesis of these membrane ruptures is discussed. The ‘1st branchial cleft’ appears in mice embryos at the age of 9.75 days (second half of ED 10), the ‘2nd branchial cleft’ appears first at the age of 10.5 days (middle of ED 11), and the ‘3rd branchial cleft’ appears at the age of 11 days (end of ED 11). In the rat, the corresponding developmental stages are seen about 24 h later. Whereas the ‘1st and 3rd branchial clefts’ are demonstrable during a period of maximally 12 h, the ‘2nd branchial cleft’ is present during a developmental period of about 24–36 h. This protracted existence of the ‘2nd branchial cleft’ is possibly related to the occurrence of lateral cervical fistulas and cysts in the region of the second branchial arch.