An experiment was conducted on 54 male Wistar rats (36 experimental, 18 controls) to study adverse effects of monosodium L-glutamate (MSG) on pancreatic acinar cells by measuring the qualitative and quantitative changes in acid phosphatase (ACPase) in these cells. Various concentrations of aqueous MSG were given to rats for various durations (Group I, 1% MSG for 1 month; Group II, 1% MSG for 1 week; Group III, 0.5% MSG for 1 week; Group IV, 0.2% MSG for 1 week; Group V, 0.1% MSG for 1 week; Group VI, 0.05% MSG for 1 week). The pancreas was then removed from each specimen for electron microscopic examination, cytochemical localization of ACPase, and cell fractionation for quantitative measurement of ACPase. Serum level of ACPase was determined in these animals before removal of the pancreas. The ultrastructural changes in pancreatic acinar cells revealed an increase in the number and size of autophagic vacuoles and various sizes of myelin figures in the cytoplasm and interstitial space. Dilatation of rough endoplasmic reticulum, swollen mitochondria, and altered zymogen granules were occasionally found. These changes occurred most often in Group I rats, which ingested 1% MSG for 1 month. The changes became less prominent as the dose of MSG administered was lessened. A reaction product of ACPase could also be seen in autophagic vacuoles in Group I rats. Regarding quantitative measurement, the serum level of ACPase for Group I rats was markedly higher than that of the controls and was statistically significant (20.52 +/- 4.92 vs. 12.62 +/- 4.13 U/L; p < 0.05)(ABSTRACT TRUNCATED AT 250 WORDS)