An animal model for study of the circadian rhythm in intraocular pressure (IOP) was developed using carefully maintained environmental lighting conditions. Highly predictable, precipitous increases and decreases in IOP of somewhat more than 10 mm Hg were found to be closely correlated with the onset of darkness and light, respectively. The daily spontaneous variations in IOP, once entrained, were maintained in constant darkness, but were eliminated by constant light, as is similarly seen in many mammalian systems. This model provides a novel means of investigating circadian IOP oscillation, which has been shown to be especially amplified in primary open angle glaucoma (POAG). This new rabbit model may also warrant consideration for use in screening ocular antihypertensive drugs since it could obviate the gross physiological manipulations and concomitant side effects inherent in the popular methods of inducing ocular hypertension in animal models.