The reflection coefficient (σ) and permeability (P) of urea and ethylene glycol were determined by fitting the equations of Kedem and Katchalsky (1958) to the change in light scattering produced by adding a permeable solute to a red cell suspension. The measurements incorporated three important modifications: (a) the injection artifact was eliminated by using echinocyte cells; (b) the use of an additional adjustable parameter (Km), the effective dissociation constant at the inner side of the membrane (c) the light scattering is not directly proportional to cell volume (as is usually assumed) because refractive index and scattering properties of the cell depend on the intracellular permeable solute concentration. This necessitates calibrating for known changes in refractive index (by the addition of dextran) and cell volume (by varying the NaCl concentration). The best fit was for a = 0.95, Po = 8.3 X 10-4 cm/s, and Km = 100 mM for urea and a = 1.0, Po = 3.9 × 10-4 cm/s, and Km = 30 mM for ethylene glycol. The effects of the inhibitors copper, phloretin, p-chloromercuriphenylsulfonate, and 5,5'-dithiobis (2-nitro) bensoic acid on the urea, ethylene glucol, and water permeability were determined. The results suggest that there are three separate, independent transport systems: one for water, one for urea and related compounds, and one for ethylene glycol and glycerol.