The effects of cilostazol, a selective cyclic AMP (cAMP) phosphodiesterase inhibitor, on retinal and choroidal blood flow and retinal arteriole diameter were examined in anesthetized rats. The retinal and choroidal blood flow was measured using laser Doppler flowmetry, and the diameter of the retinal arterioles was measured using digital video microscopy. Cilostazol was administered by two routes; systemically via the intravenous route, and directly into the retinal vessels via the intra-arterial route. When administered intravenously, 1 mg/kg of cilostazol produced a biphasic blood flow response, composed of an initial decrease which was dependent on a depressor response of mean arterial pressure, and a subsequent slight but significant increase which was independent of changes in mean arterial pressure. When administered intra-arterially over a 2-min period, 40-55 and 400-440 microg of cilostazol both produced an increase in the blood flow in a dose-dependent manner, while a depressor effect was observed only at the dose of 400-440 microg. The diameter of the retinal arterioles was increased after the intra-arterial injection of cilostazol (400 microg). It is concluded that intra-arterially administered cilostazol induces vasodilation of the retinal arterioles of rats, which results in an increase in blood supply to the retina, independent of changes in mean arterial pressure.