The possibility of alterations in the metabolism of glucose in the small intestine of C57BL/6J (ob/ob) obese-diabetic mice has been investigated. Glucose metabolism was assessed by direct measurement in vitro, and by assaying the activities of glycolytic and pentose phosphate pathway enzymes. The small intestine of 3-week-old obese mice exhibited a reduced glucose metabolism and hexokinase activity in comparison with lean controls. In adult animals there was little evidence for an effect of hyperphagia or hyperinsulinaemia on metabolism of glucose supplied from the lumen, except that the elevated pyruvate kinase activity in the intestinal mucosa of 20-week-old obese mice might have been a consequence of hyperinsulinaemia.