Cynomolgus monkeys were treated topically in one eye twice daily with prostaglandin F2 alpha-l-isopropylester (PGF2 alpha-IE) for nine doses. On treatment day 4, 3 hr after the seventh dose, intraocular pressure (IOP) in the treated eye was reduced by 65% compared to the controls, to less than 5 mmHg. On treatment day 5, 3 hr after the ninth dose, total outflow facility was determined by two-level constant pressure perfusion of the anterior chamber. Immediately thereafter, uveoscleral outflow was determined by intracamerally infusing [125I]- or [131I]-albumin and fluoresceinated dextran, and calculating the volume of anterior chamber fluid required to have deposited the quantity of tracer recovered from the various ocular and periocular tissues. Simultaneously, trabecular outflow was determined by calculating the volume of anterior chamber fluid required to have deposited the quantity of tracer recovered from the general circulation. Total facility was approximately 50% higher in treated than in control eyes, but the effect was variable, of marginal statistical significance, and perhaps due to increased pseudofacility or uveoscleral facility. Uveoscleral outflow was approximately two to three-and-a-half times higher in treated than in control eyes, the magnitude of the effect being dependent upon the timing and pressure at which the perfusion was conducted. Trabecular outflow was reduced by approximately 75% in the treated eyes relative to control so that the proportion of total outflow comprised by trabecular outflow in the treated eyes was only one third that in the controls. Total aqueous flow was slightly (approximately 20%) but not significantly reduced in the treated eyes. The IOP lowering effect of PGF2 alpha in the cynomolgus monkey is due largely if not exclusively to an increase in uveoscleral outflow of aqueous humor, with aqueous outflow being redirected from the trabecular to the uveoscleral route.