THE PURPOSE of this investigation was to determine the concentrations of most of those constituents of the aqueous humor of the posterior chamber, relative to those in the plasma and the aqueous humor of the anterior chamber, that contribute appreciably to the osmotic pressure of this fluid. Knowledge of the comparative concentrations of these constituents in the three fluids makes it possible to draw further inferences concerning the mechanism by which the aqueous humor is formed. Little is known concerning the nature of the aqueous humor of the posterior chamber. Coincidental to an investigation on another subject, I found that the osmotic pressures of the aqueous humor in the two chambers are the same1; this indicates that the total concentrations of dissolved substances are also very nearly the same. However, the aqueous humor of the posterior chamber was shown to contain about one and a half times as much