Plasma glucose and insulin responses and basal and insulin-stimulated glucose uptake were determined in 24 non-obese, healthy, physically active individuals, divided into two groups on the basis of age. The mean (+/- SEM) age of the younger group was 33 +/- 3 years, in contrast to an age of 64 +/- 2 years for the older group. Plasma glucose concentrations were significantly higher (two-way ANOVA, P less than .001) for three hours after a 75 g oral glucose challenge in the older group, as was the plasma insulin response (two-way ANOVA, P less than .001). Furthermore, there was a significant correlation between age and total plasma glucose (r = 0.63, P less than .001) and insulin (r = 0.44, P less than .01) during the glucose tolerance test. However, the magnitude of the decrease in glucose tolerance with age was relatively modest. For example, total plasma glucose response was only 11% higher in the older group, and the plasma glucose concentration 120 minutes after the oral glucose load only increased approximately 2 mg/dL per decade. Glucose uptake during euglycemic clamp studies was also reduced in the older group, and this was true if the clamps were performed at plasma insulin concentration of approximately 10 microU/mL (P less than .05) or 60 microU/mL (P less than .10). However the differences were relatively modest in magnitude, ie, 10-25%. The fact that the increase in glucose uptake when plasma insulin was raised six-fold was similar in both groups suggests that insulin sensitivity does not decline with age.(ABSTRACT TRUNCATED AT 250 WORDS)