An instrument is developed to measure pupil diameter from both eyes in the dark. Each eye is monitored with a small IR video camera and pupil diameters are calculated from the video signal at a rate of 60 Hz. A processing circuit, designed around a video digitizer, a digital logic circuit, and a microcomputer, extracts pupil diameter from each video frame in real time. This circuit also highlights the detected outline of the pupil on a monitored video image of each eye. Diameters are exported to a host computer that displays, graphs, analyzes, and stores them as pupillograms. The host computer controls pupil measurements and can turn on a yellow light emitting diode mounted just above each video camera to excite the pupillary light reflex. We present examples of pupillograms to illustrate how this instrument is used to measure the pupillary light reflex and pupil motility in the dark.