It is well accepted that the rise in intraocular pressure seen in both normal and glaucomatous eyes following ingestion of a substantial volume of water is accompanied by a reduction in blood tonicity.1,2This sudden change in osmolarity of the blood presumably promotes an increased rate of influx of water into the eye. It has been reported that outflow facility decreases following water ingestion and that this too may contribute to the pressure rise. This has been observed in both normal and glaucomatous eyes, but to a much greater extent in the latter.3Other authors4,5have reported little or no effect on facility after water. It has also been reported that in normal eyes there is an increase in facility following water while in glaucomatous eyes facility remains unchanged.6 All of these studies have used tonography to assess facility. It is known that tonography has many