Effect of ageing on insulinemia, determined by radio-immunological method, has been studied in male rats aged 3, 12 and During the intravenous hyperglycemia test, the coefficient of glucose assimilation decreases significantly with age, whereas a hype insulinemia is to be seen. This becomes apparent at the moment of the hyperglycemic peak and still persists 20 min after the beginning of the test. Senescence in the rat is, therefore, accompanied by early decrease in glucose tolerance and abnormally high IRI (immunoreactive insulin) level in the blood.