Oral glucose tolerance tests using a 75 gm. carbohydrate load were performed on 396 Pima Indians. Subjects were divided into groups on the basis of two-hour plasma glucose levels and the patterns of insulin response examined. Two-hour insulin levels were highest in the group with two-hour plasma glucose levels between 140-169 mg,/100 ml. and then fell progressively until levels about 400 mg./100 ml. were reached. Half and one-hour insulin levels showed little change in the groups with two-hour glucose levels up to 170 mg,/100 ml., but at higher glucose levels these insulin levels also progressively diminished. Fasting insulin levels were relatively unchanged over the entire range of glucose inintolerance. Obesity was the most important factor influencing the fasting insulin levels. Glucose level was the major determinant of post-load insulin responses, but these were also significantly influenced by obesity. No effect of age or sex on insulin levels was demonstrated. Comparison with other published data indicated that different interpretations of insulin response in subjects with "mild diabetes" have resulted from comparisons of groups with different degrees of glucose tolerance.