Cynomolgus monkeys were treated topically in one eye with 2 micrograms of prostaglandin F2 alpha -l-isopropylester (PGF2 alpha-IE) twice daily. On day 4, 3 hr after the seventh dose, intraocular pressure (IOP) was 8.1 +/- 0.7 mmHg lower in the treated than in the control eyes. On day 5, after the ninth PGF2 alpha dose, gross facility was measured in both eyes by determining the rate of fluid flow from an external reservoir into the eye at two different IOP levels. Simultaneously, trabecular facility was measured by determining the rate of accumulation of intracamerally infused radioiodinated albumin in the general circulation. Gross facility was 40-60% higher in PGF2 alpha treated than in control eyes, but there was no difference in trabecular facility. The increase in gross facility sometimes reported following PGF2 alpha probably represents increased pseudofacility or uveoscleral facility, rather than true facility of outflow through the trabecular meshwork and Schlemm’s canal.