The fasting blood sugar level and glucose tolerance were investigated in seven amle lowlanders at sea level, during their stay at an altitude of 4,000 m at intervals of 2 weeks, 10, 15, 20, and 24 months and again on return to sea level during the first week and after 1 month. For comparison, the glucose tolerance of six male Ladakhis (natives of high altitude area) was also determined at altitude. The fasting blood sugar among lowlanders increased to 136.0 plus or minus 4.39 mg per 100 ml during 10 months of stay at altitude followed with a gradual decrease to a value of 76.4 plus or minus 3.8 at the end of 24 months. On return to sea level, the blood glucose showed a tendency to increase. Ladakhis had a lower blood sugar level at altitude (86.4 plus or minus 7.28) as compared with lowlanders at sea level (92.6 plus or minus 2.29). The glucose tolerance curves of lowlanders ran paralell to each other at altitude and at sea level. However, the peak of the glucose tolerance curve shifted towards the left during the 20th and 24th months of stay at altitude and at sea level. However, the peak of the glucose tolerance curve shifted towards the left during the 20th and 24th months of stay at altitude and on return to sea level. The tolerance curve of Ladakhis was similar to that of lowlanders at altitude, but showed a sharper blood sugar decline rate.