This study was carried out to investigate the ultrastructural changes of the proximal epiphyseal plate of the femur in young chickens that had been treated with monosodium glutamate(MSG). Eighty 1-day old broiler chickens(Hubbard strain) were divided into control and experimental groups. The experimental group received daily administration of MSG(3mg/g of body weight in 0.75% saline) per orally for 1, 3, 6, 9, 12, 15, 18 and 21 days, and were sacrificed with exanguination. The control group received an equal volume of 0.75% saline. For the transmission electron microscopy, the prehypertrophic cartilage zone of epiphyseal plate was cleaved, fixed with 2% glutaraldehyde(containing 0.2% ruthenium red), postfixed with 1 % osmium tetroxide, embedded in Epon 812, and stained with uranyl acetate and lead citrate. For the scanning electron microscopy, the calcified zone of epiphyseal plate was cleaved and coated with gold palladium. The results obtained were as follows; 1. On transmission electron microscopic examination, the sacculation decreased from 12 day to 21 day MSG administrated groups, and the vesiculation decreased in 18 and 21 day MSG administrated groups in rough endoplasmic reticulum of chondrocytes in prehypertrophic cartilage zone. The ruthenium red binding particles in pericellular rim, territorial matrix and interterritorial matrix increased from 9 day to 21 day MSG administrated groups, but the crystalloid materials decreased. 2. On scanning electron microscopic examination, the trabecular formation and calcospherites of calcification zone decreased in 18 and 21 day MSG administrated groups. The resorption cavities widened from 15 day to 21 day MSG administrated groups.