The vitreous is a neural extracellular space separated from the blood-vascular compartment by the blood-retinal barrier. Study of the appearance of serum proteins in this space have been carried out in rats with streptozotocin-induced diabetes mellitus, a condition associated with barrier dysfunction. A vitreous sampling technique that avoids contamination with surrounding tissue was employed. In rats 1 month after administration of streptozotocin (fasting serum glucose > or = 375 mg/dl), significant increases in vitreous protein were observed in the absence of discernible eye pathology. Two-dimensional isoelectric focusing and SDS-polyacrylamide gel analysis of the soluble fraction demonstrated 85 polypeptides, 28 of whose electrophoretic positions coincided with positions of serum polypeptides. The remainder were unrelated to serum polypeptide loci. Overall patterns of soluble protein from the vitreous of streptozotocin-injected and normoglycemic-uninjected control animals were virtually identical. Results support a system for selective transfer for certain proteins into the extraneural vitreous space as suggested by Chen and Chen (6).