The marginal plasma zone of retinal vessels has been measured by a photographic technique. The mean arterial plasma zone of the pig retina was 9·7% of the internal diameter and the mean venous plasma zone 8·0%. The arterial and venous plasma zones increased with the diameter of the vessel. Changes in arterial oxygen tension and perfusion pressure produced changes in the arterial and venous plasma zone proportional to the changes in vessel size. With increases in arterial carbon dioxide tension, the arterial plasma zone became wider than would be expected from the increase in diameter.