The effects of timolol on terbutaline- and VIP-stimulated aqueous humor flow were investigated in cynomolgus monkeys, with a labeled albumin dilution method.The maximal increase in aqueous humor flow caused by intracameral (100μg/ml) or intravenous (0.4μg/kg/min) administration of terbutaline was about 100%. The effect of intravenously infused terbutaline was completely abolished by intracameral administration of timolol, 0.1 mg/ml. The same dose of timolol also abolished the effect of intravenously infused VIP, 50 ng/kg/min. Intravenous administration of timolol, 0.2 mg/kg, had no effect on VIP-stimulated aqueous humor flow, when VIP (90μg) was given intracamerally, but abolished completely the effect of intracameral terbutaline, 100μg/ml.The results suggest that the effect of intravenously infused VIP on aqueous humor flow is secondary to activation of the sympathetic nervous system, while the effect of intracameral administration of VIP is a direct effect on the ciliary epithelium. The maximal aqueous humor flow achieved with terbutaline is comparable to that in conscious cynomolgus monkeys.