The mechanism underlying the intraocular pressure (IOP) lowering effect of a novel prostaglandin-related compound, (+)-isopropyl Z-7-[(1R,2R,3R,5S)-3,5-dihydroxy-2-(3-oxodecyl)-cyclopentyl]hept-5 -enoate (UF-021), and the influence of this new hypotensive agent on the permeability of the blood-aqueous barrier (BAB) were studied in albino rabbits. A single instillation of 0.06% UF-021 significantly lowered the IOP with a peak reduction of 5.2 mmHg without changing the aqueous flow rate. Uveoscleral outflow and outflow facility to general blood circulation were both increased. The BAB permeability evaluated with fluorescein isothiocyanate-labeled rabbit albumin (FITC-RA) showed approximately sixfold increase after a single instillation of 0.06% UF-021, which suggested a drug-induced compromise of the rabbit BAB.