Twelve rhesus monkeys were used for an experiment in which intraocular pressure was maintained at various levels between 0 and 50 mm Hg for one hour. A quantitative light microscopic analysis was made of the changes in the vacuolation of the endothelium lining Schlemm's canal. At 0 mm Hg, giant vacuoles were not found; at 8 mm Hg the counts of vacuoles were significantly lower than the corresponding 15 mm Hg controls; at 22 and 30 mm Hg the counts were significantly higher than the 15 mm Hg counts; and at 50 mm Hg 71 counts of vacuoles were less than at 22 and 30 mm Hg. Within the range of 8 to 30 mm Hg, change in pressure was shown to produce alterations in both vacuole numbers and their dimensions. The number of vacuoles increased linearly with increase in pressure, whereas mean vacuolar volume increased in a nonlinear manner.