Viscous solution drainage from the eyes of unanesthetized albino rabbits was determined and related to drug bioavailability. When using methylcellulose as the viscosity-inducing polymer, it was shown that the rate of solution drainage is related to viscosity and decreases with increasing viscosity. Over a range of 1–15 cps viscosity of the instilled solution, there is a threefold change in the drainage rate constant and a further threefold change over the viscosity range of 15–100 cps. The decline in pre-corneal drug concentration was determined to be first order in concentration, and the rate of decline was proportional to the viscosity of the instilled solution. When using pilocarpine nitrate as the test drug in both miosis and aqueous humor drug concentration studies, it was shown that, over a range of 1–15 cps solution viscosity, there is a linear relationship between the first-order drainage rate constant and both the miotic activity and aqueous humor drug levels attained. Comparison of the present study in rabbits to reported behavior in humans suggests that the vehicle viscosity influence is qualitatively similar but quantitatively different in both species, with the quantitative difference being due, in part, to the difference in rate of blinking and precorneal solution movement in rabbits as compared to humans.