The microvascular changes in the wing of bats, Myoitis lucifugus, were observed during 3 weeks of normal life and 5 weeks of streptozotocin induced hyperglycemia (300–400 mg/dl, plasma). During normal life, week-to-week variations in diameter and blood flow in the same set of vessels were minor. After hyperglycemia was induced, the major initial response was dilation of all microvessels except the smallest arterioles which constricted. The dilation phase was followed by progressive constriction of all vessels. Blood flow was near normal during the dilation phase, but flow gradually decreased as hyperglycemia continued and vasoconstriction occurred. The ability to repeatedly observe the same set of microvessels on a week-to-week basis and the absence of anesthesia may make the diabetic bat a useful model in which to study the early phases of microvascular pathology during chronic hyperglycemia, which begins abruptly in adult life. In addition, the sequence of microvascular changes during chronic hyperglycemia in the bat are qualitatively similar to those in other diabetic mammals, including man.