Unilateral rhegmatogenous retinal detachments were produced in 14 cynomolgus monkeys. A total vitrectomy was performed in the fellow eyes. Two groups of monkeys were studied two to four months and ten to 19 months after creation of the detachment. Aqueous flow was determined fluorimetrically from decay of fluorescein isothiocyanate dextran 70 following anterior chamber injection. Outflow facility was measured by constant pressure perfusion. The intraocular pressure was 7.6 and 5.0 mm Hg lower in the detached eyes of the short- and long-term groups, respectively. Aqueous flow was reduced by 70% in the detached eyes of both groups. Outflow facility was slightly lower in the detached eyes (0.21 v 0.27 microL/min/mm Hg). Apparently, reduced aqueous humor formation accounts for hypotony in the detached eyes. However, shunting of posterior chamber aqueous into the subretinal space and across the blood-retinal barrier by a mechanism independent if IOP is possible.