Otoconial membranes from a large number of normal albino and pigmented guinea pigs were examined by light microscopy. In more than 20% of the animals, microdissection revealed that the crystalline layer of the otoconial membrane was incomplete, leaving large areas of the underlying gelatinous layer uncovered by crystals. These defects were bilateral, more or less symmetrical, and invariably more extensive in the saccule than in the utricle. In two animals, huge single conglomerates of crystals were found on both the saccular and utricular maculae. None of the animals displayed abnormal behavior or posture and no obvious cause for the otoconial defects was found. The presence of a hereditary congenital condition is postulated. The high incidence of otoconial defects in guinea pigs makes them poorly suited for most investigations of experimentally induced pathology. Therefore, previous reports of otoconial pathology in guinea pigs should be viewed in the light of these findings.
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