The concentrations of glucose in blood during fasting and during hyperglycemia induced by intravenous injection (1 g/kg glucose) were determined in 3 groups of adult rats aged 3, 12 and 24 months. The fasting glycemia is remarkably stable independent of the age of the rat (mean 86 mg/l00 ml). In contrast, the glucose up-take coefficient diminishes in a highly significant way between 3 and 12 months of age. These results show that ageing induces an early loss of tolerance to glucose in the animal, which is quite independent of the phenomena of digestion usually discussed.