• The effect of vasodilators on choroidal blood flow (CBF) was studied by the krypton clathrate Kr 85 desaturation technique in anesthetized cats. Niacin, papaverine hydrochloride, phentolamine mesylate, and tolazoline hydrochloride were injected via the lateral long posterior ciliary artery (LLPCA) and the femoral artery. The systemic administration of these drugs via the femoral artery, in the doses used, had no significant effect on mean arterial pressure, choroidal vascular resistance (CVR), or CBF. Local administration of the same agents via the LLPCA caused a decrease in CVR with a corresponding increase in CBF.