The contributions of diminished insulin sensitivity and decreased insulin response to the development of non-insulin-dependent diabetes mellitus (NIDDM) remain controversial. Nondiabetics in high-risk populations for NIDDM, including Pima Indians and Mexican Americans, are characterized by obesity and hyperinsulinemia relative to nondiabetics in the lower-risk white population. However, it is not clear to what extent diminished insulin sensitivity in the high-risk groups reflects obesity per se or is an inherent characteristic of these groups. Insulin sensitivity and secretion were determined in 10 nonobese, normoglycemic Mexican Americans (mean body mass index [BMI], 23.8 kg/m2) and 11 normoglycemic non-Hispanic whites (mean BMI, 22.5 kg/m2) using the intravenous glucose tolerance test (IVGTT) and the minimal model approach of Bergman et al. Age, BMI, sum of skinfolds, and the ratio of waist-to-hip circumference (WHR) were similar in both ethnic groups. Mexican Americans had decreased insulin sensitivity compared with non-Hispanic whites (4.06 +/- 0.72 in Mexican Americans v 7.56 +/- 1.13 in non-Hispanic whites, P = .017). The areas under the C-peptide and insulin curves were significantly greater in Mexican Americans than in non-Hispanic whites (P less than .01), suggesting greater insulin secretion in the former. This study provides evidence for diminished insulin sensitivity and increased insulin response in young, nonobese, normoglycemic Mexican Americans.