We have used scanning and transmission electron microscopy to examine adult human semicircular canals taken at autopsy and an operation. Our findings can be summarized as follows. In the semicircular circular canals obtained at autopsy from a 30-year-old male, non-sensory ciliated cells having 50-60 cilia were seen sporadically in the canal side of the ampulla of the horizontal canal. A labyrinthine specimen taken at operation demonstrated folding of the cell membrane at the base of columnar cells lining the wall of that part of the canal close to the ampulla. Distal to this site, canal wall cells became progressively flattened and their intracytoplasmic organelles were seen to be progressively less abundant. These cells were found to be layered, facing the endolymphatic space. Dense filaments and fibrocytes were present outside these cells, between which were interposed single-layered basement membranes. Since the capillary network present was found to be sparse, we believe that the portion of the canal studied does not produce endolymph but plays a role in the maintenance of the composition of endolymph and perilymph.