Rabbits underwent the single-dose or long-term therapeutic administration of the adrenergic drugs phenylephrine hydrochloride, timolol maleate, and betaxolol hydrochloride. After a single dose, all three drugs caused substantial, localized constriction in the arterioles that supply the ciliary processes but did not affect the downstream bore of the same vessels. After seven weeks of a daily dose, tolerance reduced the response to betaxolol to insignificant levels and that to phenylephrine substantially, whereas timolol maleate continued to produce identical levels of vasoconstriction to those seen with single-dose administration. In addition to the consequent lowering of perfusion of the ciliary processes and presumptive impact on aqueous humor production, vasoconstriction also reinforces concerns about impaired vascular perfusion of eyes undergoing long-term ocular therapy.