The effects of topically applied bunazosin hydrochloride, a recently developed highly selective alpha 1-adrenergic antagonist, on intraocular pressure, pupillary diameter, aqueous protein concentration, anterior chamber volume, aqueous flow rate and tonographic outflow facility were investigated in eight normal human volunteers. A single application of 0.1% bunazosin hydrochloride caused a significant unilateral reduction in the intraocular pressure from 30 minutes through 4 hours after the application, with a maximum decrease of 2.0 +/- 0.5mmHg (mean +/- SEM) below the contralateral control eyes at 2 hours post-application. Neither pupillary diameter, anterior chamber volume nor outflow facility was affected by the drug application. Both aqueous protein concentration measured with a flare-cell meter and aqueous flow rate determined by fluorophotometry were unaltered. The mechanism of intraocular pressure reduction appears to be an increase in uveoscleral outflow and/or a decrease in episcleral venous pressure.