A review of previous evidence and of his own work in which conception of the alkaline tide of the urine in the original sense of decreased ti-tratable acidity or the later sense of increased pH is contrasted with the conception of increased acid-ammonia output per unit of time. The latter conception allows a numerical relation to be sought between the acid of the urine and other body changes: the former is inferior in this respect. Reference is made to various methods of measuring the urinary acid. Digestive activity may be accompanied by a decreased acid output in the urine. This has usually been held to depend on the loss of acid by the blood during the production of gastric juice. Food may reduce the urinary acid output either through excess of its basic radicles, through preferential absorption in time of its alkali constituents, by the oxidation in the body of its organic acid, or by saving protein metabolism. Voluntary hyperpnea can produce a fall of CO2 in the blood and a decreased acid output in the urine. The latter is accompanied during sleep by a rise in the output of phosphates and sometimes by a fall in the COa-combining power of the blood. Evidence is not available for a complete description of the phenomena. A fall of blood CO2 and a decreased acid output occur immediately on waking. Their cause is not completely understood but they are certainly not due to food and probably not due to getting out of bed.