The permeability of frog skin to a series of nonelectrolytes (thiourea, urea, mannitol, and sucrose) under the influence of 2.5 times normal osmolarity in the outer bathing solution has been investigated. Although the flux of the tracer nonelectrolytes across the skin in either direction is greatly increased by hyperosmolarity, the influx is found to be increased to a significantly greater extent than the outflux. Flux ratios as high as 3:1 can be observed. The net inward movement of the nonelectrolyte proceeds in spite of a sizeable bulk flow of water in the opposite direction. Possible driving forces for this phenomenon are discussed.

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