Changes in the transparency and size of lenses in rats were investigated following administration of monosodium-L-glutamate (MSG), MSG (5 mg/g b.w.) was injected subcutaneously on the 9th and 10th days after birth. Morphological changes of the lenses were examined ophthalmoscopically and histologically. Ophthalmoscopically, localized opacity was apparent at the posterior pole of the lens corresponding to the attached portion of hyaloid artery at 1 months of age. The incidence of cataract increased with age, reaching more than 75% at 4 months of age. Morgagni's globules were histologically detected in the opacity of the posterior lens cortex. Degenerative changes of the lens epithelium were observed in the mature cataract. However, the capsules of the lens were well preserved. The size and weight of the lens were smaller than those of controls. Although the pathogenesis of the cataract and small lenses are not clear at the moment, these findings indicate that administration of MSG could be an etiologic factor in cataract formation in the developing rat.