The anterior chamber of adult pigmented rabbits was perfused with 1 ml of either 0.02% or 0.2% thymoxamine hydrochloride in Balanced Salt Solution. Contralateral eyes were either perfused with BSS alone or remained untouched. Measurements were made of corneal thickness and intraocular pressure prior to perfusion and at days 1, 2, 3 and 7 after perfusion. Observations were made of toxic reactions of ocular tissues using slit-lamp biomicroscopy. No effects were noted with 0.02% thymoxamine but 0.2% caused corneal swelling and a reduction in intraocular pressure at day 1. Neither concentration caused any observable toxic responses. Morphologic studies confirm the absence of effects by 0.02% thymoxamine, and that 0.2% causes disturbance of both corneal endothelium and iris-ciliary processes. This data, together with an earlier report on in vitro studies of thymoxamine, allows the conclusion that 0.02% thymoxamine is a safe concentration for use in the anterior chamber and 0.2% yields adverse reactions.