Glucose intolerance has been observed often in elderly subjects, but it is not yet clear whether this impaired metabolic state is due to the aging process itself or is secondary to the appearance of other age-related variables. This study attempts to elucidate the effect of age in itself on factors controlling glucose tolerance. Several metabolic parameters were measured in 10 young male controls (23-29 yr) and 17 nonhospitalized, healthy, nonobese, old (60-80 yr) male subjects. Insulin binding to circulating cells was performed along with the intravenous glucose tolerance test, and the data were analyzed by the minimal model method. This approach yields the following measures: tissue insulin sensitivity (SI), fractional glucose disappearance at basal insulin (glucose effectiveness, SG), and first (phi 1) and second (phi 2) phase beta-cell responsiveness to glucose. Insulin-binding capacity to monocytes and erythrocytes was respectively 6.03% +/- 0.57% and 5.96% +/- 0.53% (elderly), 5.97% +/- 0.39% and 5.36% +/- 0.57% (young); SI was 6.20 +/- 0.59 X 10(4) min-1/(microU/mL) (elderly) and 6.35 +/- 0.30 (young); SG was 0.016 +/- 0.002 min-1 (elderly) and 0.019 +/- 0.003 (young); phi 1 was 1.84 +/- 0.29 min-1 (microU/mL)/(mg/dL) (elderly) and 3.37 +/- 0.84 (young); phi 2 was 13.80 +/- 1.78 X 10(4) min-2 (microU/mL)/(mg/dL) (elderly) and 9.59 +/- 2.65 (young). These results show no change with aging of tissue insulin sensitivity and an intact beta-cell activity, suggesting that age per se does not contribute to the deterioration of glucose tolerance when the effect of other age-related variables, eg, obesity and physical inactivity, is precluded.