Insulin secretion and glucose disappearance rate were measured in 66 subjects with a wide range of fasting plasma glucose levels. The acute insulin response was present in subjects with fasting glucose levels below 115 mg/dl but was absent above this level. The glucose disappearance rate related to the relative acute insulin response in subjects with fasting glucose below 115 mg/dl and to total insulin response when fasting glucose levels were above 115 mg/dl. A calculated glucose disappearance rate of 1.06 per cent per minute was found when the acute isulin response was zero. All subjects with fasting glucose levels greater than 115 mg/dl had glucose disappearance rates greater than 1.06. These studies support 1) epidemiological data indicating 115 mg/dl as an upper limit of normal for fasting plasma glucose levels and 1.0 per cent per minute as a lower limit of normal for the glucose disappearance rate, and 2) evidence for an important role for the acute insulin response in the determination of glucose disappearance rates during intravenous glucose tolerance tests.