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 $15.7{pm}3.4,;48.3{pm}9.8,;40.4{pm}6.7,;37.4{pm}6.5;and;26.0{pm}4.2uU/ml(Mean{pm}S.E.)$ and C-peptide were $1.9{pm}0.3,;3.9{pm}0.6,;6.3{pm}0.6,;5.7{pm}0.5;and;4.0{pm}0.5ng/ml$. 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 $38.9{pm}12.3,;59.5{pm}12.3,;59.2{pm}17.1,;56.1{pm}20.0;and;48.4{pm}17.2uU/ml$ and the C-peptide were $5.5{pm}0.4,;6.8{pm}0.5,;7.9{pm}0.8,;7.9{pm}0.8;and;7.8{pm}2.0ng/ml$. The insulin response appeared to be greater than nonobese normal subjects. 3. In 13 nonobese moderate diabetic patients, the plasma insulin levels were $27.1{pm}4.9,;44.1{pm}6.0,;37.3{pm}6.6,;35.5{pm}8.1;and;34.7{pm}10.7uU/ml$ and the C-peptide levels were $2.7{pm}0.4,;4.9{pm}0.7,;6.5{pm}0.5,;7.0{pm}0.3;and;6.7{pm}1.0ng/ml$. 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 $22.1{pm}7.9,;80.0{pm}19.3,;108.0{pm}27.0,;62.0{pm}17.6;and;55.5{pm}10.1uU/ml$ and the C-peptide levels were $5.2{pm}0.4,;8.0{pm}1.0,;10.4{pm}1.6,;10.4{pm}1.7;and;10.1{pm}1.0ng/ml$ and its response was also greater than that of nonobese moderate diabetic patients. 5. The plasma insulin concentrations in 9 severe diabetic subjects were $8.0{pm}3.8,;12.1{pm}3.5,;16.8{pm}4.6,;19.6{pm}5.2;and;15.0{pm}5.0uU/ml$ and the C-peptide levels were $1.6{pm}0.3,;2.4{pm}0.4,;4.1{pm}0.6,;4.0{pm}0.8;and;4.5{pm}0.7ng/ml$ 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.