The effects of topical application of a single dose of prostaglandin F2 alpha, administered as the isopropylester, on the intraocular pressure (IOP), aqueous humor flow (AHF), conventional, and uveoscleral outflow were studied in cynomolgus monkeys under pentobarbital anesthesia. 1 microgram PGF2 alpha decreased the IOP by 2.9 +/- 0.6 mmHg (3 hr after the application) as compared with the vehicle-treated control eye. The mean AHF during the whole experiment was slightly higher in the experimental than in the control eye, 1.34 +/- 0.11 microliters min-1 compared with 1.16 +/- 0.09 microliters min-1. The uveoscleral outflow was significantly increased in the PGF2 alpha-treated eye, 0.98 +/- 0.12 microliters min-1 compared with 0.61 +/- 0.10 microliters min-1 for the control eye. The conventional outflow was lower in the experimental eye throughout the experiment. Topical application of 10 micrograms pilocarpine at the time when the fall in IOP was expected prevented the drop in the IOP. Simultaneously the increase in the uveoscleral outflow was abolished. After systemic pretreatment with atropine, 1 mg (kg body weight)-1 i.v., there was no significant difference in IOP, AHF, conventional or uveoscleral outflow between the PGF2 alpha-treated, and the control eye. The results of the present investigation suggest that PGF2 alpha decreases the intraocular pressure by increasing the uveoscleral outflow. The mechanism behind the increase in the uveoscleral outflow remains to be established. Relaxation of the ciliary muscle as well as enlarged intramuscular spaces and loss of extracellular material may contribute to the effect.