Plasma glucose levels before and after oral glucose administration have been compared in a group of 76 thyrotoxic subjects and a group of 8 normal control subjects in order to study the effect of glucose loading in thyrotoxicosis. Following were the results: 1. The mean fasting plasma glucose level was elevated in the thyrotoxic group(95.5mg%) compared to normal control group (88mg%). 2. The peak of glucose tolerance curve is at 30 minutes after glucose administration in both groups, but its mean value was 44mg% higher in thyrotoxic group than in control group. 3. The plasma glucose levels returned towards the fasting level in the later stage of the test more rapidly in thyrotoxic group than in control group. 4. 69.6% of oral glucose tolerance tests were impaired in the thyrotoxic group, and the occurance of abnormal glucose tolerance could be related to the degree of thyrotoxicity, sex and age. 5. The mechanisms of the impaired glucose tolerance in thyrotoxicosis are thought to be related to an increased rate of glucose absorption from gastrointestinal tract, abnormal liver function with decreased hepatic glycogenesis, increased glucose oxidation, decreased pancreatic release of insulin, and genetic relationship between diabetes and thyrotoxicosis.