WITHIN less than 2 yr, two diametrically conflicting views of the ionic permeability of the guinea-pig taenia coli muscle have been published in Nature1,2. The first1 assumed by analogy with frog skeletal muscle3–5 that the membrane is permeable to Cl. The intracellular Cl was then calculated from Donnan considerations and the values so obtained seemed to agree quite well with estimates based on electrophysiological measurements of inhibitory postsynaptic potentials. The second report2 does not refer to the first, but it confirms still earlier data6 on the “anomalous” osmotic behaviour of taenia coli muscles. The muscles do not gain weight on prolonged exposure to isosmotic KCl-enriched media. They lose weight when exposed to media that are made hyperosmotic by addition of KCl.