Circadian rhythms in serotonin metabolism have been observed in the pineal gland and retina, and there is evidence that the levels of serotonin and melatonin in these tissues may mediate events in the brain's sleep-wake cycle and the retina's cycle of disc shedding. Because the ciliary epithelium, which produces aqueous humor, has an embryonic origin similar to that of the retina and the pineal gland, the authors believe that serotonin metabolism might play an analogous role in the regulation of the diurnal cycle of aqueous secretion. As a first step in investigating this hypothesis, they measured serotonin concentrations in the aqueous humor of 34 cataract patients. Using high-performance liquid chromatography (HPLC) with electrochemical detection, a mean serotonin concentration of 52 +/- 11 ng/ml was found. This high concentration implies that serotonin has one or more functions: (1) perhaps it acts as a neurotransmitter and precursor of melatonin; (2) conceivably it is related to aqueous humor dynamics; and (3) in particular, perhaps it affects the circadian rhythm of intraocular pressure.