Introduction Clinical evaluation of the ocular circulation has relied primarily upon subjective analysis of the retinal and choroidal vasculature. Attempts at objective study have been made using opthalmodynamometry and arm-to-retina circulation times. Neither of these tests is directly related to the ocular blood flow. It is the purpose of this communication to describe a technique which allows a direct comparison of the blood flow rates of the two eyes in man. The blood flow rate in a given region can be measured directly by timing the speed of filling with new blood. The rate of appearance of the blood is proportional to the blood flow rate through the vascular bed. The blood in the retinal and choroidal circulations can carry an identifying marker whose passage can be timed by optical means through the transparent preretinal media of the eye. We have developed an apparatus which measures the appearance of fluorescein