A circadian rhythm of intraocular pressure in rabbits could provide a useful model for understanding the daily rhythm of intraocular pressure in humans and for studying mechanisms which regulate intraocular pressure. Our results confirm earlier work showing that New Zealand White rabbits housed in an environment with a lighting cycle of 12 hours light and 12 hours dark have a rhythm of intraocular pressure, and that this rhythm persists in constant dark. We show further that the cycle of light and dark is thezeitgeberfor entrainment of the rhythm of intraocular pressure, and therefore persistence of this rhythm in constant dark establishes it as a circadian rhythm. Cervical ganglionectomy demonstrated that intact sympathetic innervation to the eye is required for maintenance of the normal circadian rhythm of intraocular pressure in rabbits. Intraocular pressure in sympathectomized eyes is no different from control eyes during the light, but is significantly reduced during the dark.