It has been hypothesized that insulin might mediate meal-related diurnal variation in vaspin serum concentrations. We therefore investigated whether insulin affects serum vaspin levels in humans. Vaspin serum concentrations were determined by ELISA in 10 healthy individuals, who underwent an insulin tolerance test (ITT) for the evaluation of pituitary ACTH and growth hormone reserve. The ITTs were started 08:00 am after an overnight fast with a bolus i.v. insulin dose of 0.15 IU/kg body weight (Actrapid). Blood samples were taken at -15, 0, 15, 30, 60, 90, and 120 min after insulin administration. 15 min after insulin administration, vaspin serum concentrations decreased by 19 ± 6%, continued to decrease by 42 ± 12% at 60 min and returned to 88 ± 7% of initial values 120 min after insulin administration. Our data suggest that meal-related changes in serum vaspin concentrations might be mediated by insulin.