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The activities of all living cells are so profoundly influenced by the hydrogen ion concentration of their surroundings that for a number of years physiologists have concerned themselves very extensively with determinations of the pH of physiological fluids without sufficient recognition of the fact that two solutions of exactly the same pH may have totally different effects upon the internal reaction of cells exposed to them. It has been shown by Jacobs (1-3), in the case of systems containing CO2 and bicarbonate or free ammonia and ammonium salts, that external and internal reactions frequently do not run parallel. Intracellular acidity may be produced by alkaline solutions, or intracellular alkalinity by acid ones. The experiments of Smith and Clowes (4, 5) also show that while the reaction of the surrounding sea water is a factor of much importance in determining the rate of cleavage of A slerias and Arbacia eggs, the effects produced at the same pH are very different according to whether the buffer system used contains much or little free CO2. Very recently, similar results have been obtained with rabbit intestine by Fraser (6) and with human leucocytes by Pearse (7).In the present series of experiments an attempt has been made to determine whether the same principles apply to cilia, which because of their small volume and relatively enormous surface might be expected to show less contrast between external and internal effects than is the case with most other protoplasmic structures. The general result of these experiments is to indicate that, while the pH 693