Summary. The metabolism of circulating disaccharides was studied in adult humans and rats. After iv infusions of 10 g of either lactose, sucrose, or maltose in four adults, no rise in blood glucose was noted. A mean of 8.7 + 1.89 g of the lactose and 6.3 + 1.39 g of the sucrose was excreted in the 24-hour urine sample. Only 0.11 ± 0.03 g of the infused maltose was recovered in the urine, suggesting that the maltose was metabolized.After injection of 14C-labeled lactose and sucrose in rats, 6.2 + 2.7 and 7.6 ± 2.4%, respectively, was oxidized to 14CO2 in 24 hours; 62.1 ± 13.5 and 68.4 ± 10.8% of the respective disaccharides was excreted into the urine. Conversely, after injection of 14C-labeled maltose 54.6 + 7.0% was oxidized to 14CO0 and 4.8 + 3.9% excreted in the urine. The per cent of maltose oxidized to CO2 was similar to that of glucose.In addition to small intestinal mucosa, homogenates of rat kidney, brain, and liver as well as serum were found to have measurable maltase activities.