The mydriatic effects of ibopamine solution at the concentrations of 0.25%, 0.50%, 1% and tropicamide solution at the concentration of 1% were evaluated in 22 clinically healthy volunteers. The study was conducted at single dose, instilling 1 drop of each concentration of ibopamine and of tropicamide in one eye and 1 drop of placebo in the other. The pupillary diameter was measured by photographic pupillography immediately before and at 10, 20, 30, 45, 60, 90, 120, 180, 240 min after each instillation. Immediately before and 45 min after each instillation, visual acuity was also measured for both near and distant objects. The ibopamine solution exerted a dose-related mydriatic effect in both magnitude and duration. The mean maximum pupillary diameters were 5.5, 8.0, 9.1 mm with ibopamine at 0.25%, 0.50%, 1% and 8.6 mm with tropicamide. The mean times to reach the maximum pupillary diameter were 60, 54, and 50 min with the three concentrations of ibopamine, and 47 min with tropicamide. The duration of the mydriatic effect was less than 4 h with ibopamine and very much more with tropicamide. No effect on accommodation was found with ibopamine, while a cycloplegic effect was constantly noted with tropicamide. On the basis of the results obtained, it is concluded that ibopamine is to be preferred when mydriasis is required with good absolute values and brief duration (examinations of the fundus, examinations with Goldmann's lens, etc.), without cycloplegia.