Permeability of the anterior segment of adult Wistar-Furth rats to fluorescein-labeled dextrans (FITC-dextrans) was evaluated by fluorescein angiography following intravenous injection of 0.2 ml of a 33% FITC-dextran solution. Permeation of each fluorescent compound into the anterior chamber was evaluated by (1) initial appearance of fluorescence in iris vessels, (2) the appearance of fluorescence at the pupil, (3) contrast between the iris vessels and their surroundings, and (4) disappearance of FITC-dextrans from the iris blood column. Permeability of FITC-dex 3 (molecular weight, 3000; effective diffusion radius, 12 A) was similar to that of sodium fluorescein in that both entered the aqueous rapidly and showed diffuse aqueous fluorescence within 3 min. As the molecular weight of the FITC-dextrans increased (m.w. of 20,000, 40,000, 70,000, and 150,000 were used), the FITC-dextrans exhibited decreased aqueous permeation, and iris vessel contrast persisted. For FITC-dex 150 (m.w. 150,000; 85 A) no fluorescence was visible in the pupil at times as long as 1 hr. Since these fluorescent compounds exhibit aqueous permeability properties based upon molecular size and thus are similar to blood proteins, they should be useful for in vivo and in vitro quantitative study of the blood-aqueous barrier in animals.