The mydriatic effects of tropicamide (Mydriacyl), hydroxyamphetamine hydrobromide (Paredrine), phenylephrine, hydrochloride (Neosynephrine Hydrochloride), and homatropine hydrobromide in concentrations normally used in clinical ophthalmology were studied in 15 subjects, pupillary diameters being measured with the infrared electronic pupillograph. Graphs and important characteristics of the mydriasis-time data for each subject and each drug were obtained by use of the computer. It was found that tropicamide had the shortest latency period and produced the greatest pupillary dilatation in the shortest time of the four drugs. Homatropine and phenylephrine produced similar amounts of mydriasis, with the effect of homatropine lasting a much longer time. Hydroxyamphetamine was the least effective. The mydriatic response of subjects with light irides was greater than the response of those with dark-colored irides.