The permeability of the blood-vitreous barrier to fluorescein passing out of the vitreous does not necessarily equal the permeability to fluorescein passing into it. We calculated the ratio between the outward and inward permeability coefficients of the blood-vitreous barrier in eight normal men who ingested 3-g of sodium fluorescein. The calculation was based on the ratio between the serum free fluorescein and the vitreous fluorescein concentrations (as determined by fluorophotometry) when the net transport across the barrier was zero. The outward permeability to fluorescein was 31 +/- 18 times (mean +/- SD) the inward permeability. To our knowledge, this article provides the first direct evidence for a specialized transport mechanism in humans whereby fluorescein is removed from the vitreous into the blood. The malfunction of this process may be important in human diseases. Pharmacologic manipulation of this process may be possible.