The present study was undertaken to evaluate the significance of the insulin and the C-peptide rseponse to oral glucose loads in normal and diabetic subjects and to establish the effects of the obesity. In this study, the authors have measured plasma insulin and C- peptide by means of radioimmunoassay in 10 nonobese normal, 5 obese normal, 13 nonobese moderate diabetic patients, 9 obese moderate diabetic patients and 9 severe diabetic patients. The results obtained were as follows; 1) In 10 nonobese normal subjects, the plasma insulin level at fasting state and at 30, 60, 90, and 120 min after oral glucose loads were 15.7¡¾3.4, 48.3¡¾9.8, 40.4¡¾6.7, 37.4¡¾6.5 and 26.0¡¾4.2uU/ml(Mean¡¾S.E.) and C-peptide were 1.9¡¾0.3, 3.9¡¾0.6, 6.3¡¾0.6, 5.7¡¾0.5 and 4.0¡¾0.5ng/ml. The change of C-peptide was found to go almost parallel with that of insulin and the insulin value reaches to the highest level at 30 min whereas C- peptide reaches to its peak at 60min. 2) The plasma insulin level in 5 obese normal subjects were 38.9¡¾12.3, 59.5¡¾12.3, 59.2¡¾17.1, 56.1¡¾20.0 and 48.4¡¾17.2 uU/ml and the C-peptide were 5.5¡¾0.4, 6.8¡¾0.5, 7.9¡¾0.8, 7.9¡¾0.8 and 7.8¡¾2.0ng/ml. The insulin response appeared to be greater than nonobese normal subjects. 3) In 13 nonobese moderate diabetic patients, the plasma insulin levels were 27.1¡¾4.9, 44.1¡¾6.0, 37.3¡¾6.6, 35.5¡¾8.1 and 34.7¡¾10.7uU/ml and the C-peptide levels were 2.7¡¾0.4, 4.9¡¾0.7, 6.5¡¾0.5, 7.0¡¾0.3 and 6.7¡¾1.0ng/ml. There was little significance compared to nonobese normal groups but delayed pattern is noted. 4) In 9 obese moderated diabetic patients, the plasma insulin levels were 22.1¡¾7.9, 80.0¡¾19.3, 108.0¡¾27.0, 62.0¡¾17.6 and 55.5¡¾10.luU/ml and the C-peptide levels were 5.2¡¾0.4, 8.0¡¾1.0, 10.4¡¾1.6, 10.4¡¾1.7 and 10.1¡¾1.0ng/ml and its response was also greater than that of nonobese moderate diabetic patients. 5) The plasma insulin concentrations in 9 severe diabetic subjects were 8.0¡¾3.8, 12.1¡¾3.5, 16.8¡¾4.6, 19.6¡¾5.2 and 15.0¡¾5.0uU/ml and the C-peptide levels were 1.6¡¾0.3, 2.4¡¾0.4, 4.1¡¾0.6, 4.0¡¾0.8 and 4.5¡¾0.7ng/ml and the insulin and C- peptide responses were markedly reduced in severe diabetic groups. 6) There were significant differences between each groups of patients on the magnitude of total insulin or C-peptide areas, the insulinogenic index and the C-peptide index.