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 < .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.