Dapiprazole HCL is currently available in the United States for reversal of diagnostic mydriasis. The recommended dosage for this indication is 2 drops followed 5 minutes later by 2 drops. We studied the dose-response profile and tolerance of three different treatment regimens: 1 drop alone, 1 drop followed by 1 additional drop 5 minutes later, and 2 drops followed by 2 additional drops 5 minutes later. Sixty normal male and female volunteers between 18 and 40 years of age were recruited for a double-masked, placebo- controlled, crossover study in which each eye of each subject was dilated with 2.5% phenylephrine. After one hour, one eye was treated with 0.5% dapiprazole, and the contralateral eye was treated with placebo. Each subject was treated with each of the three dapiprazole regimens in three different study sessions separated by at least 5 days (drug half-life in the eye is 5 hours). Analysis of AUC demonstrated no significant difference (P=0.620) between the 1 drop regimen and the 2+2 regimen. A single drop of dapiprazole has a clinical effect equivalent to the 2+2 drop regimen in eyes dilated with 2.5% phenylephrine, which should improve the cost-effectiveness of this mydriolytic agent.