The effects of timolol, epinephrine, and acetazolamide on the rate of flow of aqueous humor through the anterior chamber of awake and sleeping human subjects was studied. Timolol reduced the rate in awake subjects but not sleeping subjects. Epinephrine increased the rate in sleeping subjects to a greater extent than in awake subjects. Acetazolamide reduced the rate of flow in awake subjects or epinephrine-stimulated subjects. Acetazolamide reduced the rate of flow slightly below the basal rate observed during sleep, but the reduction was small and not statistically significant. The authors propose that the diurnal fluctuation of the rate of aqueous humor flow in humans is driven by changes in the concentration of endogenous epinephrine available to the ciliary epithelia.