The effect of topical administration of 3-isobutyl-methyl-xanthine (IBMX), a potent phosphodiesterase inhibitor, was studied on an experimentally provoked uveitis in rabbits. After presensitization with an intravitreal injection of human serum albumin (HSA), intravenous antigenic challenge induces blood-aqueous barrier breakdown and leukocyte infiltration. The effect of IBMX on the blood-aqueous barrier was determined by scoring the severity of the flare in the anterior chamber and by determination of the levels of ascorbic acid and protein in the aqueous. Treatment with IBMX 1% two times daily, significantly inhibited the breakdown of the blood-aqueous barrier and the increase in PGE2level of the aqueous humor. There was no effect on leukocyte infiltration. The therapeutic effect of IBMX in blood-aqueous barrier protection is comparable with the effect of topical treatment with the corticosteroid medrysone.