Large amounts of blood capillaries consisting of vascular endothelium, pericytes, and interstitial collagen fibers can be prepared from the swimbladder of the common eel. This unique preparation has been employed to determine the glucose utilization by the capillary tissue in vitro. In a medium containing 5 mM glucose, the glucose uptake averages 25 μmol/g wet weight per hour. Most of the uptake is accounted for by aerobic glycolysis; the incorporation of glucose carbon into CO2, glycogen, total lipids, and proteins is quantitatively minor. All the glucose pathways are stimulated at various rates when the glucose concentration in the medium is raised from 5 to 30 mM. The addition of beef insulin to the medium does not modify glucose and alanine utilization. When capillaries are isolated from hyperglycemic alloxan-treated eels, the glucose uptake, lactic acid production, and glucose oxidation to CO2 compare with those of control capillaries and do not respond to fish insulin added in vitro. The results indicate that the rate of glucose utilization by isolated blood capillaries is not limited by the cell membrane permeability but depends, at least in part, on the glucose concentration in the extracellular medium.