The present study investigates the use of the afferent pupillary reflex as an objective index of a subject's ability to detect a dim flash. The results support the general notion that the pupil responds to flashes near the absolute threshold for vision. In contrast to several previous studies, however, our results document that minute pupillary constrictions occur in response to small diameter (i.e., 3.5 degrees and 1.2 degrees ) stimuli. These pupillary constrictions occur mostly on the trials when the subject also reported seeing the flash and little evidence of a response is found on the trials when the flashes were not detected. We infer that the sensitivity of the perceptual and pupillomotor pathways are very similar for the scotopic conditions tested.