The dimensions of the membranous labyrinth partly determine the mechanical operation of the semicircular canal system. This study provides, for the first time, extensive measures in individual specimens of the sizes, cross-sectional shapes and areas of the horizontal semicircular duct, ampulla and utricle in the rat and the guinea pig. The membranous labyrinths were fixed in Karnovsky's fixative, exposed, photographed, sectioned, oriented perpendicular to the line of sight and then measured using a calibrated graticule in the eye piece of an operating microscope. As well as the expected size differences between these species, there are major differences in the shape of the utricle.