ABSTRACT: Beta-cell function was studied in young (20–30 years old, n=10) and old non-diabetic subjects (n=305), newly detected (n=87) as well as established (n=101) non-insulin dependent diabetic subjects, aged 60–75 years. Fasting plasma insulin concentrations were 0.04 ± 0.01, 0.07 ± 0.04, 0.11 ± 0.07 and 0.11 ± 0.07 pmol/ml (mean ± sd) in the different groups. The corresponding C-peptide values were 0.36 ± 0.07, 0.63 ± 0.36, 0.98 ± 0.40 and 0.95 ± 0.45 pmol/ml, respectively. The relative changes in insulin and C-peptide concentrations were identical despite different kinetics of the metabolism of the peptides, indicating, that the elevated insulin levels of old as opposed to young non-diabetics and diabetic as opposed to non-diabetic subjects are caused by hypersecretion rather than decreased metabolic clearance of insulin.