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It has been well documented that glucose intolerance is associated with aging, but it is not yet clear whether this phenomenon is due to the aging process itself or is secondary to the appearance of other age-related conditions among which physical inactivity is one of most important variables. To evaluate the effect of aging process and/or physical inactivity on insulin action, this study was undertaken using the euglycemic insulin clamp technique and the oral glucose tolerance test (OGTT). Subjects without diabetes mellitus and other serious diseases consisted of 14 non-obese aged individuals and 10 young controls (YC group), ranging in age from 63 to 85 yrs, and from 19 to 21 yrs, respectively. The aged individuals were further divided into two groups (one was termed as the AS group, in which 7 aged subjects had been confined to bed for at least 3 months and the other was termed as the AC group in which 7 aged controls kept their daily physical activity such as walking). The results of OGTT did not show any remarkable differences between AC and YC groups. In the AS group, however, glucose intolerance and low insulin response during OGTT were observed. In view of the tissue insulin action, MCR, which is thought as a reliable marker for tissue insulin action, evaluated by euglycemic insulin clamp was 5.31 +/- 0.68, 8.57 +/- 1.20, 9.60 +/- 0.35 ml/kg/min in the AS, AC and YC groups, respectively (AS less than AC, p less than 0.05, AS less than YC, p less than 0.01, AC less than YCM, N.S.).(ABSTRACT TRUNCATED AT 250 WORDS)