In the course of an investigation on the mechanism of reabsorption of the intraocular fluid1 it was found that the ciliary body shows an irreciprocal permeability to water, allowing fluid to pass with ease from the ciliary capillaries to the posterior chamber but obstructing the passage of fluid in the reverse direction. Phenomena of this type have been demonstrated in many other tissues.2 Hitherto, however, no adequate physical explanation of such phenomena has been suggested. The present investigation was undertaken with the aim of discovering, if possible, the character and mechanism of the irreciprocal permeability of the ciliary body. In brief outline, it was first discovered that the ciliary body shows an irreciprocal permeability to certain dyestuffs as well as to water and that the behavior of the dyes indicates that an electrical phenomenon is involved. This led us to an investigation of the source of