An instrument has been developed that measures in ten seconds the level of fluorescein along a scanning line from the retina to the cornea. With spatial resolving power of 1.1 mm in the eye, concentrations of 1 X 10(-8) gr/ml fluorescein and higher are detectable. In this way an indication of the integrity of the blood-ocular barriers is achieved. In uveitis the dysfunction is mainly localised at the blood-aqueous barrier while in diabetic retinopathy the blood-retinal barrier is predominantly affected. Sources of error are discussed.