Morphine's miotic action on the pupil is an easily recognizable and quantifiable effect in man. The neural pathways responsible for regulating pupil size are reasonably well defined. Yet, the mechanisms behind this and related effects of opioids on the eye in humans and laboratory animals have just begun to be explored. In this review, we have attempted to organize the available information on pupillary actions of opioids, emphasizing the dynamic nature of the responses, their species specificity, possible mechanisms of action, and the recently discovered development of tolerance to these actions. Our current knowledge regarding differences among the opioids, the effects of endogenous opioid peptides and the role of the various opiate receptor subtypes in pupillary effects is also summarized.