Extensive morphometric measurements were made on the vestibular system of the rabbit (Oryctulagus cuniculus), the gerbil (Meriones unguiculatus), the chinchilla (Chinchilla laniger), and the squirrel monkey (Saimiri sciureus) from serial sections of temporal bones. Additionally, a more limited set of measurements were also completed on the owl monkey (Aotus trivirgatus), the Capuchin monkey (Cebus sp.), the harp seal (Pagophilus groenlandicus Erxleben, 1777), and the two-toed sloth (Choloepus sp.). The following measurements were made: (1) radius of curvature (R) of each membranous semicircular canal (herein called semicircular duct—see nomenclature in Nomina Anatomica (1968)) (2) cross-sectional diameter of the ducts and the osseous semicircular canals, and (3) some pertinent morphometrics of the cristae ampullares and the utricle. In all species studied (1) the radii of curvature of the three semicircular ducts are dissimilar, with that of the lateral duct being as small as, or smaller than, those of the anterior and posterior ducts; (2) R for the anterior duct is largest in the harp seal and the rabbit; (3) the canal and duct dimensions are largest in the Capuchin and squirrel monkeys, the two-toed sloth, and the harp seal, and smallest in the gerbil; (4) the proportion of otic fluid “space” that is occupied by endolymph shows a ranking of gerbil > rabbit > two-toed sloth > chinchilla = owl monkey > squirrel monkey > Capuchin monkey > harp seal; and (5) the gross ampullary and utricular dimensions are largest in the harp seal and smallest in the gerbil. These measurements were used for determining the time constants describing semicircular-canal dynamics in the Steinhausen (1931, 1933) and Oman-Marcus (1980) equations.