The effect of retrobulbar epinephrine administration on ocular and optic nerve blood flow was studied in phakic and aphakic rabbit eyes using a radioactive microsphere (85Sr) technique. One hour before blood flow determination, 10 microliter of a 0.2% (base) epinephrine bitartrate solution was administered to the right eye of each experimental rabbit by the retrobulbar route. For control rabbits, 10 microliter of saline was injected retrobubarly. Blood flow measurements were determined for the iris, scrap[ed ciliary processes, choroid, retina, and optic nerve. Neither optic nerve nor ocular blood flow was altered by this dose which is approximately the dose used in retrobulbar injections in humans.