To evaluate the glucose tolerance and insulin secretion in hyperthyroidism patients were examined in the toxic state and after they had been made euthyroid. Fasting values: In 42 untreated patients the glucose- and insulin concentrations in serum were significantly elevated. In 24 treated patients the glucose concentrations became normal, while the insulin concentrations remained elevated. Oral-glucose-tolerance test: In 20 untreated patients the glucose- and insulin responses were significantly increased. In 8 treated patients the glucose response became normal, while the insulin response remained unchanged. Intravenous-glucose-tolerance test: In 28 untreated patients the K-values were significantly decreased and the insulin response increased. In 23 treated patients the K-values rose significantly, but the insulin response remained unchanged. Intravenous-tolbutamide test: In 41 untreated patients the glucose concentration decreased significantly compared with the controls, and the insulin responses were significantly increased. In 23 treated patients the glucose concentrations decreased even more, while the insulin response remained unchanged. The results indicate enhanced sensitivity or an increase in the mass of beta-cells in hyperthyroidism. The glucose tolerance tests point to an increased peripheral insulin resistance. The normalized glucose tolerance and still enhanced insulin secretion during treatment support the assumption, that hyperthyroidism causes an increase in the beta-cell mass.