Rabbits entrained in a daily light-dark environment show a circadian elevation of intraocular pressure (IOP) around the onset of dark. It was reported that concentration of cyclic AMP (CAMP) in aqueous humor increases significantly during this time period. Whether or not the increase of ocular CAMP-mediated activities is related to the circadian elevation of IOP is unclear. Light-dark entrained rabbits maintain the circadian IOP elevation in a constant dark environment. Corresponding to this IOP elevation in the constant dark, no change of aqueous humor cAMP concentration was found. Thus, the circadian IOP elevation in the constant dark is probably unrelated to the CAMP-mediated activities. This finding also suggests that a significant portion of the circadian IOP elevation in the light-dark environment is unrelated to the CAMP-mediated activities.