Levitt (1974) (Biochim. Biophys. Acta 373, 115--131) has recently developed a "New Theory of Transport for Cell Membrane Pores" based on the supposition that equivalent pores in the red cell membrane are so small that water and small solute molecules such as urea can not pass each other. Levitt's concept is based on the implicit assumption that urea and water are spherical molecules. We have shown, using a scale model, that Levitt's supposition is not in agreement with the actual molecular shapes. Levitt has further asserted that there is a serious methodological error in measurements reported fifteen years ago by Goldstein and Solomon (1960) (J. Gen. Physiol. 44, 1--17). We have shown that the supposed "methodological error" lies in the fact that Levitt made his mathematical analysis of the appropriate equations under conditions significantly different from those employed by Goldstein and Solomon. A computer solution of the equations under the actual conditions used shows that Levitt's assertion is not justified.