Introduction No methods have yet been devised which allow direct measurement of aqueous humor flow. Indirect methods employed presently make use of the equation as described by Friedenwald.1 It can be seen from the equation that if K (rigidity), P2 (final pressure), and P1 (initial pressure) are known or can be determined, then ΔV (volume change, flow) can also be ascertained. In studies which were designed to investigate the influence of brain stimulation on aqueous outflow, inconsistent results were obtained when this equation was used. In view of the importance of the pressure-volume relationship to tonometry and tonography in estimating aqueous outflow, it was felt advisable to reexamine this relationship. Methods Twenty-seven young adult cats (1.7 to 2.5 kg.) served as experimental animals in this series. They were anesthetized with chloralose (20 mg/1 cc. of 75% alcohol) 2.0 cc. per kilogram of body weight, administered intraperitoneally. Scleral