After a screening visit, participants were scheduled for 2 study visits: 1 daytime visit (day 1) and 1 nighttime visit (day 3, Table 1). The night before the daytime measurements, starting between 10 PM and midnight, participants self-administered 1 drop of fluorescein sodium, 2%, in each eye every 5 minutes, for a total of 6 to 8 drops. At 9 AM, central corneal thickness was measured by ultrasound pachymetry, anterior chamber depth by A-scan (Pacscan Series 300; Sonomed, Lake Success, New York), and seated IOP by pneumatonometry (Model 30 Classic; Reichert Ophthalmic Instruments, Depew, New York). Aqueous flow was determined during the next 3 hours by fluorophotometry,15 using an ocular fluorophotometer (Fluorotron Master; OcuMetrics, Mountainview, California) every 45 minutes to measure the fluorescence of the cornea and anterior chamber. At noon, IOP was measured and oral acetazolamide, 500 mg, was administered. One hour later, fluorophotometry was performed, and repeated for 2 more measurements, from approximately 1 PM to 2:30 PM; IOP was measured after each set of afternoon scans. Outflow facility was calculated at each of the 3 measurements as the ratio of the change in aqueous flow to the change in IOP. The mean of the 3 values was the reported outflow facility.16 At approximately 2:30 PM, outflow facility was measured by 2-minute tonography, using the tonography setting on the pneumatonometer. Seated blood pressure was measured by sphygmomanometry at noon. Episcleral venous pressure was measured by an episcleral venomanometer (Eyetech, Morton Grove, Illinois) at 10 AM. The episcleral veins were identified as those that did not move when the probe was applied and were deeper than conjunctival vessels. Some vessels were seen to contain aqueous humor. Vessels were observed near the temporal limbus. The pressure behind the probe was increased until the vessel half-collapsed, then the pressure was reduced slightly to let the vessel refill and then increased again until half-collapse again. That was the point at which the pressure was recorded.17 If the difference between the 2 measurements was more than 2 mm Hg, a third measurement was taken. With the exception of tonography, all measurements during the daytime visit were made with the participant in the seated position.