We used a double-masked, randomized, placebo-controlled study of the effect of acetazolamide alone and timolol maleate alone and of the two in combination on the rate of aqueous humor flow in 21 normal subjects. The rate of aqueous flow was measured by fluorophotometry over an eight-hour period. Timolol decreased the flow by 33%, acetazolamide by 27%, and the drug combination by 44%, compared to the placebo treated eye (P less than .01). Timolol produced a 26% decrease in intraocular pressure. Timolol decreased the intraocular pressure in the fellow eye by 16%, acetazolamide by 21%, and the two drugs together by 35% (P less than .01). Neither drug significantly affected the anterior chamber volume, the endothelium's permeability, or the estimated endothelial pump rate.