A single-drop, double-masked, randomized, placebo-controlled study of the mechanism of the ocular-pressure lowering property of timolol maleate, a beta-adrenergic blocker, was carried out in 23 normal subjects, using fluorophotometry. Timolol suppressed aqueous formation in all subjects. The range of suppression was 13% to 48%, with a mean +/- SD of 34% +/- 9%. The drug had no effect on anterior chamber volume or endothelial permeability to fluorescein and, apparently, had no effect on outflow resistance. No differences were observed between its effect on men and women or between eyes with light and dark irides.