The effect of exogenous prostacyclin (PGI2) on the rabbit eye was investigated. During intravenous infusion, intraocular pressure readings and changes of the luminal diameters of choroidal veins were recorded simultaneously. 5 ng/kg/min induced a reduction in intraocular pressure and no blood flow changes. 25 ng/kg/min resulted in a considerable vasodilatation in the posterior uvea paralleled by an increase in intraocular pressure. These results suggest the explanation that exogenous prostacyclin in high doses causes an augmentation of posterior uveal blood flow and thereby a rise in intraocular pressure. Low (2.5 mg/kg BW) or high (17.5 mg/kg BW) aspirin caused no changes in intraocular pressure, indicating a minor role of endogenous prostaglandins in regulating intraocular tension.