When female eels, fasting and sexually mature, were progressively adapted to cold water (2-4 degrees C), their blood sugar concentration rose to values averaging 600 mg/dl. Control eels, kept in warm water (18-20 degrees C), had a mean blood sugar concentration of 100 mg/dl. After a period of 5-6 mo, the blood capillaries of the rete mirabile in the swimbladder were examined in both control, low blood sugar eels, and in cold-adapted, high blood sugar eels. In the latter, the basal laminae of the capillaries were thickened; their amino acids composition was altered and the in vitro glucose carbon incorporation into basal laminae glycoproteins was increased over a wide range of medium glucose concentrations. Furthermore, the diffusion capacity of the rete, as measured with tracer molecules during steady-state conditions in a countercurrent perfusion system, was increased in the hyperglycemic eel. It is concluded that chronic hyperglycemia in the cold-adapted eel is associated with a microangiopathy characterized by morphologic, biochemical, and functional alterations.