In a placebo-controlled prospective study, thymoxamine 0.02% or acetylcholine 1.0% or thymoxamine 0.01% plus acetylcholine 0.5% were used intraocularly to reverse mydriasis during cataract surgery. Prior to surgery, pupils were dilated with scopolamine 0.25%, tropicamide 1%, and phenylephrine 10%. At the end of surgery, 228 eyes were randomly assigned to 1 of 4 different treatments: 57 eyes each received intracamerally thymoxamine 0.02%, acetylcholine 1%, a combination of both drugs, or placebo. At 5 min posttreatment, there was a significant mean pupillary constriction of 1.86 +/- 0.67 mm following acetylcholine administration and 1.53 +/- 0.80 mm following thymoxamine application as compared with the placebo group, in which the pupillary diameter remained almost constant. For the combination of acetylcholine plus thymoxamine the effect was fully additive, with a pupillary constriction of 3.42 +/- 0.98 and 4.04 +/- 1.02 mm being observed at 5 and 10 min posttreatment, respectively. No drug-related side effect was detected. We conclude that thymoxamine can be useful during surgery as an intraocular miotic to reverse mydriasis and can considerably enhance miosis when given in combination with acetylcholine.