A method is described for measuring the segmental mean circulation time and estimating the segmental flow in the human retina. Serial retinal photographs were taken after intravenous fluorescein injection and the relative concentration of fluorescein in the superior temporal artery and vein calculated from measurements of the optical density of the vessel images. Arterial and venous time-concentration curves were constructed and the mean circulation time calculated. In normal subjects the mean circulation time in the superior temporal segment varied from 1·39 to 6·85 sec. This variation was largely explained by variations in the volume of the vascular bed which was taken to be related to the sum of the square of the arterial and venous diameters. Volume flow showed less than a two-fold variation in normal subjects, mean 55 ± SD 11 units/sec. Similar results for volume flow were found in hypertensive subjects, although flow was more variable in these patients, mean 54 ± SD 18 units/sec. Some anaemic patients showed a marked increase in volume flow, and there was a significant relationship between packed cell volume and retinal volume flow (R = 0·65, P = 0·01).