A new photostimulator and an infrared videopupillograph are described. The light output from the photostimulator is quantitatively variable, and the beam of light is focused in the plane of the pupil to a spot smaller than the narrowest pupil, thus affording open loop conditions. The videopupillograph gives a direct and continuous measure of the area of the pupil. The importance of these features in quantitative neurophysiological studies is discussed.