The main object of this paper is to discuss the large differences in concentration of individual ions between the interior of living cells and the fluids surrounding them, to bring out the point that such differences are normally, and perhaps always, brought about and also maintained by some special activity on the part of the cell, while diffusion processes are all the time tending to reduce them, and are, when maintained, expressions of steady states , differing in principle from equilibria by the necessity of energy being supplied for their maintenance. The pertinent facts are usually discussed under the heading ‘permeability’ (Wilbrandt 1938; Davson & Danielli 1943; Blinks 1942; Faraday Soc. Discussion 1937), but it will be shown below that, although permeability of the cell surface is of course a necessary corollary of the ion transport taking place, quantitative determinations of such permeabilities, in the generally accepted sense of the term, can be made only by means of isotopes and even then require special precautions and conditions which are difficult to realize and verify, because the exchanges normally taking place are largely brought about by active transport. The argument is concerned only with ions of strong electrolytes which are responsible in many cases for almost the whole of the osmotic pressure in organisms and cells, while in others they make up at least a large fraction. Other substances will be dealt with only incidentally and for the sake of comparison. Experimental results from the most diverse sources are brought together for discussion. The experiments carried out by myself and my associates make up only a minor part of the whole, and even some of these have been published before. Forced to leave Denmark about the middle of 1944 I had to discontinue experimental work, which had already suffered severely from the difficulties encountered. I have been fortunate in finding a refuge at the Institute of Physiology in Lund, and I wish to express my deep gratitude for the kindness shown me. Everything has been done to provide facilities and papers for my work, but many recent publications have proved inaccessible in Sweden.
