This reviews in concise form the most important facts regarding variations of p H in health and in disease. Only work done with modern technique and of importance in chemical medium is included. New experiments concerning the relation of Ca to p H in tetany show that with injection of alkaline or acid phosphate, low Ca with normal pa caused tetany but the same low level of Ca with decreased p a does not cause tetany. p H influences susceptibility of organisms to Ca-ion change or to change in balance of inorganic ion. Evidence is given showing that the condition called post operative acidosis is probably not an acidosis. Additional information concerning temperature effects on the p h and CO2 curves of the blood is given.