In the experimental study of the relation between size and distance, little consideration has been given to exposure-time as a determinant of size-estimates. In a recent study, however, Leibowitz, Chinetti, and Sidowski did include exposure-time as a variable in the investigation of size-constancy. They illuminated a hallway for intervals of 0.0005 sec. and found that estimates made under this condition were "essentially identical with those obtained under continuous illumination, demonstrating that reduced exposure-time has little effect on the perception of size."' In that experiment a fixation-point of light was presented at the standard before the standard was illuminated, and such other cues to distance as perspective and interposition were not eliminated during its illumination. The present experiment was undertaken to determine what effect limited exposuretime might have on size-constancy under conditions similar to those reported by Chalmers2 and by Holway and Boring.3 Cues to distance in the present investigation were limited to those of accommodation, convergence, and retinal disparity. Observers. Each of 12 Os, aged 20-34 yr., met a visual acuity criterion of 1.0 (corrected vision) on a minimum-separability test. Each was naive concerning the purpose of the experiment and the theoretical aspects of constancy phenomena. Apparatus. The experiment was conducted in a darkened tunnel 130 ft. long. Five cut-out triangles were used as standards, one being placed at 20, 30, 40, 50, and 80 ft. from O. The altitude of each standard subtended a visual angle of 1?. By remote control, E could move any standard in and out of O's field of vision. Concealed lighting on a white panel at the end of the tunnel produced a homogeneous reflectance of 3.0 ft.-L and provided illumination for the cut-out triangles. A comparison-triangle, geometrically similar to the standards, was set 20 ft. from O and to the left of the standards (3? separation, base mid-point to base mid-point). Illuminated to match the brightness of the white panel, the comparison was variable in altitude from 0? 12' to 4? 48'. The walls of the tunnel were so curtained that the triangles appeared in a totally darkened field. An episcotister permitted exposures of 0.1 and 0.8 sec. For trials on which either of these two brief exposures was used, a manually operated shutter enabled E to limit O to single, rather than repeated, exposures. The shutter was used also to present exposures of 4.0 sec.