1. The effect of acidification of the medium on the rate of oxygen consumption of aquatic organisms was studied. 2. The acids used were: hydrochloric, nitric, sulphuric, carbonic, butyric, acetic, citric, and tartaric. They were added to water to produce acidities ranging from pH 7.5 to 5.0, at intervals of 0.5 pH. 3. Planaria dorotocephala was the chief animal used as material. Some tests with hydrochloric acid were also made using starfish and nudibranchs. 4. The acidification of natural waters, either salt or fresh, (pH 8.0), by any of the acids used except butyric causes a decrease in the rate of oxygen consumption at all acidities greater than pH 7.0. The majority of acids also cause depression between 7.8 and 7.0 but in the case of acetic acid and more doubtfully carbonic, there was some tendency towards a slight acceleration of the rate of oxygen consumption at these lower concentrations. 5. The decrease in the rate of oxygen consumption due to acids is completely and promptly reversible, as long as the animals are not actually injured. 6. The acidification of fresh water from which all carbonates have been previously removed has no or only a slight effect upon the rate of oxygen consumption of Planaria, except when the acidity is produced by carbon dioxide. 7. The depressing action of carbon dioxide is the same whether the gas is added to ordinary or to carbonate-free water. 8. From 6 and 7 it follows that the depressing action of acids in natural waters is due chiefly or wholly to the carbon dioxide which they liberate from the carbonates of such waters. 9. The depressing action of acidified natural waters on the rate of oxygen consumption of Planaria is not the same with different acids at the same hydrogen ion concentration. This appears to be due largely to the fact that the amount of carbon dioxide immediately liberated from the carbonates of the water differs with different acids at the same pH. 10. The depressing action of acids in natural waters is greater the greater the acidity up to an acidity of about pH 5.0. This is due to the fact that the more acid added, the greater is the quantity of carbon dioxide liberated. 11. Lower concentrations are, however, relatively more effective than higher ones. 12. The maximum amount of depression of oxygen consumption that can be induced by acids is about 50 per cent. This occurs at pH 4.0 to 5.0 and further acidification of the water does not increase the percentage of depression. At the acidity at which the maximum depression appears, the carbon dioxide content is about 3 per cent. 13. All of the carbonates of the fresh water employed are decomposed by acids at a pH of 4.0 to 5.0, producing a carbon dioxide concentration of 3 per cent. This might explain the facts given in 12 were it not that concentrations of carbon dioxide gas much higher than 3 per cent. do not increase the percentage of depression beyond 50 per cent., as long as the oxygen supply is ample. Concentrations of carbon dioxide gas up to 25 per cent. were tested. 14. From the facts cited in 13 it appears necessary to assume that the depression of the rate of oxygen consumption which can be induced by carbon dioxide does not exceed 50 per cent. as long as the oxygen supply is adequate. 15. A combination of high carbon dioxide content and low oxygen content practically abolishes the oxygen consumption of Planaria, even though the oxygen content used (2 cc. per liter) would be ample for normal respiration in the absence of carbon dioxide. 16. No explanation has been discovered for the differences between the action of carbon dioxide and other acids at low concentrations (pH 7.5 and 7.0). 17. Butyric acid has almost no action on the oxygen consumption of Planaria. No explanation has been discovered for this fact. 18. The different acids employed are lethal for Planaria in a given arbitrarily selected time (2 to 3 hours) at different hydrogen ion concentrations. The order of toxicity and the hydrogen ion concentrations at which the acids are equally lethal are: butyric (5.0), acetic (4.4), tartaric (3.6), citric (3.4), sulphuric (3.2) and nitric and hydrochloric (3.0). 19. The facts given in 18 prove that the hydrogen ion is not the cause of death but either the anion or the molecule of the acid is involved. Penetrability is probably also a factor. Death in acids appears to be due to coagulation. 20. The order of toxicity of the acids and the pH at which they are equally lethal are the same in ordinary and in carbonate-free water, showing that death is not due to carbon dioxide liberated. 21. Acidification of natural waters constitutes a method for depressing the rate of oxygen consumption of aquatic animals for experimental purposes. 22. The experiments herein presented cast doubt on the supposed importance of hydrogen ion concentration per se in biological processes.