Glucose tolerance tests were performed with fourteen cynomolgus monkeys. They were divided into two groups with regard to the serum glucose level at the time of routine health-examination. Nine of them had normal glucose level (below 123 mg/dl, the normal group) and the other five monkeys exhibited hyperglycemia (the abnormal group). Fifty per cent glucose solution was administered into the saphenous vein at a dose of 4 ml/head. Blood samples were taken just before and 5, 10, 20, 30, 60 and 120 minutes after the glucose administration. K-value (K = 0.693/t 1/2 X 100) as the decreasing rate of serum glucose during from 5 to 60 minutes after the administration was calculated. Average K-value for eight monkeys of the normal group was 3.12 +/- 0.48. Both immunoreactive insulin level (IRI) and C-peptide immunoreactivity (CPR) increased just after the glucose administration and began to decrease 5 to 30 minutes after the administration in all the eight animals. Remaining one animal (No. 009) of the normal group showed 1.03 in K-value. For the abnormal group, K-value averaged 0.75 +/- 0.25. IRI was slightly higher in this group than in the 8 monkeys of the normal group. Furthermore, the abnormal group did not show any definite change of a certain trend in IRI and CPR. In conclusion, the former 8 monkeys were judged to be normal in the function of pancreatic beta-cells, and the latter 5 monkeys and No. 009 monkey were judged to be suffering from type II (noninsulin dependent) diabetes mellitus at different stages of the disease.