• Mean circulation time (MCT) of sodium fluorescein in retinal vascular segments was determined in healthy human volunteers, using the dye dilution technique. The fluorescein dilution curves were recorded with a two-point fluorophotometer. Average MCT was 3.9 seconds. The accuracy of the method was determined from repeated measurements in owl monkeys and human volunteers. No significant linear correlation was found between the MCT and the cross section of the vessels feeding and those draining the segment. The application of this technique to clinical work is discussed, and is illustrated by three clinical cases.