This study was performed on 35 male wistar rats to demonstrate the effect of monosodium L-glutamate (MSG) on the pancreatic acinar cells. MSG in a aqueous solution was ingested with food ad libitum. The ultrastructural alterations in pancreatic acinar cells revealed increased number and size of autophagic vacuoles, various sizes of myelin figures in cytoplasm and interstitial space, and the aggregation of small size myelin figures at the cell base. Dilation of rough endoplasmic reticulum and swollen mitochondria with distorted cristae were also found in rare cases. Altered zymogen granules with loss of peripheral electron density were usually demonstrated in experimental rats and controls. The number ultrastructural alterations were found to be proportional to the dosage of MSG ingested and duration of administration. The rats in group VI which ingested the largest doses for the longest period of time experienced the most prominent changes. On the other hand, the group V rats ingested the smallest doses and had almost no ultrastructural changes in their acinar cells and the ultrastructures of their acinar cells were not distinguishable from those of the controls. The levels of serum amylase were not significantly higher than those of the controls, implying that MSG only caused a subclinical pancreatic acinar cell injury in this study