Intravenous glucose tolerance tests were made on male Sprague-Dawley rats, raised on different diets. Two natural diets (McCollum's wheat-casein ration and table scraps) produced glucose removal rates of around 4%/min. or more. In rats raised on three commercial pelleted stock rations (Purina, Hunt Club, Rockland) as well as on a semipurified Torula yeast diet, low glucose removal rates of 2.5–2.8%/min. were consistently detected after 20 or more days of feeding. The results are related to the contents, in the investigated diets, of the glucose tolerance factor (GTF), a recently described, water soluble dietary agent of low molecular weight. Development of low glucose removal rates is prevented by dietary supplementation with brewer's yeast, or with GTF concentrates from brewer's yeast or pork kidney powder. The fully developed impairment is cured within 1–2 weeks by the addition of GTF concentrates to the diet. It is also reconstituted to normal within 18 hours after stomach tubing of GTF fractions of widely different degrees of purification. This curative procedure is being used as a 24-hour test for GTF.
