To determine whether abnormalities in glucagon secretion might precede the onset of hyperglycemia in diabetes mellitus, 32 prediabetic Pima (American) Indians, 27 normal Pima Indians and 34 normal Caucasians received an infusion of arginine monochloride (5 mg/kg/min for 40 minutes) with measurement of glucose, insulin, and glucagon. [Prediabetes is the period between conception and the development of diabetes. In most studies the term is used to characterize patients who on genetic grounds are believed to be at high risk of developing the disease, including the normoglycemic monozygotic co-twin of a diabetic or the normoglycemic offspring of two diabetic parents. The latter definition is used in the present study recognizing that in the final analysis the true prediabetic can be identified only in retrospect after the development of diabetes.] The three groups had similar mean fasting glucagon levels. During arginine infusion, the prediabetic Indians reached a mean maximum glucagon level of 315 +/- 14 pg/ml (mean +/- 1 SEM) compared with 294 +/- 20 pg/ml in the normal Indians and 292 +/- 25 pg/ml in the normal Caucasians. The calculated mean areas above baseline under the glucagon curves were 5704 +/- 324 pg-min/ml in the prediabetics, 5189 +/- 446 pg-min/ml in the normal Indians, and 4239 +/- 613 pg/min/ml in the normal Caucasians. The differences among the groups in these variables were not statistically significant. Thus, arginine induced hyperglucagonemia could not be identified as a characteristic of the prediabetic state in Pima Indians.