Eleven subjects with normal eyes and sixteen subjects with open-angle glaucoma had a selective alpha-adrenergic blocking agent, thymoxamine hydrochloride 0.5 per cent eyedrop, applied to one of their eyes. This consistently produced miosis, but did not significantly alter the intraocular pressure or the tonographically determined facility of aqueous outflow. Because we find that thymoxamine can selectively induce miosis without significant accompanying effect on the ciliary muscle or facility of outflow, we expect that this drug may prove to be a useful diagnostic adjunct to gonioscopy in distinguishing between angle-closure glaucoma and open-angle glaucoma with narrow angle. It seems worthwhile proceeding with clinical evaluation of this possibility.