Anterior chamber paracentesis disrupts the blood aqueous barrier (BAB) of rabbits and nonhuman primates, but the magnitude and duration of breakdown in monkeys has not been clarified. We have studied anterior chamber paracentesis in cynomolgus monkeys as a potential model of postoperative BAB breakdown. The effect of a single paracentesis upon fluorescein sodium concentration in the anterior chamber after an intravenous injection was measured in 16 eyes of 8 animals. In an additional 10 eyes of 5 animals, aqueous humor was withdrawn for analysis 24 hours and one week following paracentesis. Anterior chamber fluorescein concentration was 57 +/- 22 ng/ml (mean +/- standard deviation) before paracentesis, rose to 81 +/- 47 ng/ml 24 hrs after paracentesis, and was 60 +/- 36 ng/ml at 72-96 hours. Twenty-four hours after paracentesis, total protein concentration was elevated, but ascorbic acid and transforming growth factor-beta levels were not. Paracentesis in monkeys has only a small and short lasting effect upon BAB integrity and is therefore unlikely to be a good model for assessing the effect of agents designed to stabilize the BAB. However, the short-lived effect of paracentesis may permit the repetitive collection of "primary aqueous" for physiologic and biochemical studies.