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Background: Insulin resistance of the peripheral tissue can be induced by high-fat diet in rats. Glycogen synthase activity is controlled by insulin. Recently, many researchers have investigated glycogen synthase activity in various insulin resistant states. However, glycogen synthase activity of different skelatal muscles in high-fat diet induced insulin resistance is not established. The purpose of our study was to know the effects of high-fat diet on the glycogen synthase activity of the different skeletal muscles in rats. Methods: We used 13 female Wistar rats weighing 150 to 200 grams. Rats were randomly assigned into normal (fat: 5%) and high-fat (fat: 35%) fed rats. Glycogen synthase activity of different skeletal muscles were meaured after 4 weeks of feeding. Experimental procedures were carried out following overnight fasting. Skeletal muscles used were gastrocnemius red as fasttwitch red, gastrocnemius white as fast-twitch white, and soleus as slow-twitch red. Results: Gain (gm) in body weight was higher in highfat diet rats (55+9.4) compared to normal diet rats (45+7.6). Plasma glucose level (mmol/L) was higher in highfat diet rats (7.0+0.7) than that of normal diet rats (5.3+0.7). Plasma insulin level (pmol/L) tended to be higher in high-fat diet rats (96+39) than that of normal diet rats (60+ 30). Plasma FFA (uEq/L) showed no difference between high-fat diet rats (522+53.7) and normal diet rats (474+ 53.9). Glycogen content (mg/gm wet weight) of the different skeletal muscles showed no differences between high-fat diet rats (13.1+8.4, 10.5+3.8, 11.3+6.9 in FW, FR, and SR, respectively) and normal diet rats (12.2+6.1, 10.1+5.5, and 13.2+7.8 in FW, FR, and SR, respectively). Glycogen synthase activity (dpm/mg protein) in normal diet rats was not significantly different among different skeletal muscles, but in high fat diet rats, that of FW was higher than that of the other muscles. Glycogen synthase activities of the FR and the SR muscles were lower in high-fat diet rats (9.8+8.87 and 7.6+2.09 in FR and SR, respectively) compared to normal diet rats (33,6+7.75 and 20.5+8.40 in FR and SR, respectively), but not in FW muscle. Conclusion: Insulin resistance was induced by high-fat diet in rats. Plasma glucose level was higher in high-fat diet rats than that of normal-diet rats. Glycogen synthase activity was decreased in FR and SR of high-fat diet rats compared to normal-diet rats.