Serum immunoreactive insulin (IRI) levels were higher in older persons initially and throughout a three-hour glucose tolerance test when 176 persons from a random sample of Sudbury, Massachusetts were tested. Multiple regression analyses indicate that the higher IRI levels are associated with the rise in blood glucose and body weight that accompanied advancing age. Consequently, the different frequency distributions of interdependent variables in various studies probably explain the contradictory results in the literature when the blood insulin changes with age are interpreted. It is concluded from this study that advancing age is accompanied by an adequate responsiveness of islet cell tissue to glucose and that the higher blood glucose levels are the primary alteration to occur. This implies an alteration of the insulin release mechanism, presumably affecting the stimulus required to initiate this complex since higher blood glucose levels are required to produce equivalent blood insulin levels when age increases.