The effect of topical timolol on the fluorophotometrically determined aqueous humor formation rate in the cynomolgus monkey was studied. A timolol dose of 2.5, 5, 15, 30, 45, 90, or 180 $mUg was administered as a single topical dose to one eye of five monkeys. Postdrug flow rates were compared with no-drug baselines obtained on separate occasions. Our findings were that (a) topical timolol decreases aqueous humor formation rates in a dose-dependent fashion, with 30 $mUg and 50% flow suppression at or near the top of the single dose-response curve; (b) there is a significant contralateral effect, and only at extremely low doses can treated versus control eye effects be separated; and (c) doses as small as 2.5 $mUg can suppress aqueous formation by a statistically significant 20%. In the monkey, as in the human, timolol is a far more potent suppressor of aqueous formation than is generally realized, and standard clinical doses may in fact be overdoses in both species.