The effectiveness of 1% pilocarpine in counteracting the mydriatic effect of phenylephrine hydrochloride (Neo-Synephrine Hydrochloride), hydroxyamphetamine hydrobromide (Paredrine), tropicamide (Mydriacyl), and homatropine hydrobromide in the concentrations ordinarily used in clinical ophthalmology was studied in 15 subjects. Pupillary diameters were measured with the infrared electronic pupillograph. It was found that the mydriatic effects of the sympathomimetic drugs (phenylephrine and hydroxyamphetamine) are rapidly counteracted by pilocarpine, usually in much less than 30 minutes. However, after mydriasis with parasympatholytic drugs (tropicamide and homatropine), pilocarpine produced little effective miosis and the pupil remained dilated or semidilated for a prolonged period.