This chapter discusses glucose tolerance test, which is done in order to assess the tolerance of a person for a load of ingested glucose. Glucose tolerance test is done by measuring the plasma glucose concentration and testing urine for glucose at fasting and various time intervals, after giving a glucose load. The patient is asked to come fasting overnight and his fasting blood sample and urine are collected. Then the patient is given seventy five grams of glucose orally and blood and urine samples are collected at intervals of half an hour, one hour and one and a half hours. The plasma glucose concentration in all these blood samples is estimated by the glucose oxidase method. After the oral glucose intake, there is intestinal absorption of the glucose which increases the plasma glucose level to about one hundred and forty milligrams per decilitre, one hour of taking glucose which is less than the renal threshold.