Rabbits have circadian rhythms of intraocular pressure (IOP) and aqueous flow. The dark phase increases of IOP and flow are associated with increased aqueous cyclic AMP. If the daily rhythm of aqueous cyclic AMP reflects changes of tissue levels of cyclic AMP which mediate one of the mechanisms which control the rhythms of IOP and aqueous flow, then it must be a circadian rhythm as are the rhythms of IOP and flow. A recent report showed no change in aqueous cyclic AMP from 2 hrs before to 2 hrs after the beginning of subjective dark in animals housed in constant dark. Because the increase of intraocular pressure at the same times did persist in constant dark, the authors concluded that a significant portion of the circadian rhythm of IOP is unrelated to cyclic AMP mediated ocular mechanisms. However, if an inhibitor of cyclic nucleotide phophodiesterase is added to aqueous samples after harvesting by paracentesis, it is possible to show that most of the increase of aqueous cyclic AMP from mid-light phase to mid-dark phase persists in constant dark and is therefore likely to reflect a circadian rhythm of tissue cyclic AMP.