The responses in plasma glucose, insulin, C-peptide, glucagon and somatostatin to an oral glucose load were studied in 10 thyrotoxic patients and 10 matched euthyroid controls. The thyrotoxic patients had higher mean fasting plasma glucose (P less than 0.05) and responded to oral glucose with an earlier peak at 30 min which was higher than the corresponding glucose level in the controls (P less than 0.05). Impaired glucose tolerance was found in 3 patients. Fasting insulin and C-peptide levels were normal in the thyrotoxic patients when corrected for the higher glucose levels. Following glucose ingestion, there was no significant difference between the areas under the insulin or C-peptide curves in patients and controls, but Seltzer's insulinogenic index was reduced in the patients (P less than 0.01) suggesting an impaired pancreatic B-cell response to oral glucose. Mean basal glucagon was normal in the thyrotoxic patients. However, while in the controls plasma glucagon became suppressed following glucose ingestion (P less than 0.0001), no significant suppression was found in the patients. In the thyrotoxic patients, mean basal somatostatin was normal, but the area under the somatostatin curve following glucose ingestion was significantly increased (P less than 0.02). Our findings suggest that decreased glucagon suppression and impaired insulin response after glucose ingestion are involved in glucose intolerance in thyrotoxicosis. Enhanced somatostatin responses to oral glucose in thyrotoxicosis may have contributed to the observed impairment in pancreatic B-cell responsiveness.