We measured choroidal blood flow in cats by determining the rate of clearance of indocyanine green dye from the choroidal circulation. Measurements were made by rapidly injecting indocyanine green dye into the common carotid artery and photoelectrically monitoring its concentration in the superior anterior ciliary vein. We verified the validity of our method by measuring the indocyanine green dye clearance from an animal perfused with saline through the common carotid artery to eliminate recirculation. The curve was carried out past the time of the recirculation artifact and was accurately described by a monoexponential. Moreover, we found an inverse relationship between choroidal blood flow and intraocular pressure between approximately 15 and 100 mm Hg a fact well documented by other methods. Finally, sympathetic nerve stimulation, known to decrease choroidal blood flow, had a similar effect on our flow values measured by dye clearance. Choroidal vascular resistance is defined as the perfusion pressure (mean systemic blood pressure minus intraocular pressure) divided by the choroidal blood flow. In normal cats, the choroidal vascular resistance was transiently decreased by an abrupt increase in blood glucose level induced by the intravenous injection of glucose. The maximum reduction in choroidal vascular resistance occurred at approximately three minutes and averaged 40±4 percent (mean ± S.E.). This transient reduction in choroidal vascular resistance may play a role in the pathogenesis of diabetic retinopathy.