Assay of glycosylated hemoglobin provides reliable information on metabolic control in diabetes mellitus over a period of about 90 days. This is why it is currently used as a parameter of blood glucose control in diabetic patients. However, at present little is known about the kinetics of stable glycosylated hemoglobin variations as a result of circadian changes in blood glucose level. The authors describe a mathematical model which allows to foresee glycosylated hemoglobin variability as a result of alterations of blood glucose equilibrium.