Several types of otoconia are present in the otolithic membrane of the utricle of fetal, young and adult guinea pigs. These include smooth, transitional and rough otoconia, and a few rhombohedrons. The smooth otoconia are nearly the same numeric fraction in all the specimens. The mean size of otoconia increases only during the course of gestation. Our measurements indicate that the diameter (d) and the length (l) of otoconia are related linearly as: d = 0.4l + 0.4. The smooth otoconia seem to be independent morphological form which is not derived from more "primitive", rough or transitional ones. The otoconia consist mainly of CaCO3 and contain Na, Mg and trace amount of K. The chemical composition of otoconia is similar in fetal, young and adult guinea pigs.