Aqueous humor pilocarpine hydrochloride concentration was measured at 15, 30, 60, and 120 minutes after instillation in the rabbit eye of a radioactively labeled 2% pilocarpine solution in various vehicles. Maximum concentrations were found with a vehicle containing 1.67% polyvinylpyrrolidone and other water-soluble polymers (Adsorbotears) and 1% hydroxypropyl methylcellulose. These produced pilocarpine concentrations during the first two hours after administration three times greater than those found with either saline, 1.4% polyvinyl alcohol, or 0.5% hydroxypropyl methylcellulose. Benzalkonium chloride, 0.01%, enhanced pilocarpine penetration into the aqueous by as much as 50% at all time intervals. The vehicle viscosity was unrelated to efficacy of pilocarpine penetration into the aqueous. The equally effective 1% hydroxypropyl methylcellulose and Adsorbotears have viscosities of 112 and 20 centistokes, respectively, while 0.5% hydroxypropyl methylcellulose (13 centistokes), 1.4% polyvinyl alcohol (3 centistokes), and sodium chloride solution (0.85 centistokes) are much less efficacious.