Introduction .Hydrogen is the most abundant element in the body.When bound to carbon, it usually will not dissociate, but it often does so when bound to oxygen, nitrogen or sulfur.Hence, acid-base groups are ubiquitous in biochemistry.Furthermore, it must be appreciated that water provides the environment in which most biochemical reactions occur and water is itself both an acid and a base by virtue of the fact that it can dissociate to a very slight degree as follows:H 2 O + H 2 O⇆H 3 O + + OH - Therefore, in aqueous solutions hydrated protons (hydro-nium ions) and hydroxyl ions are always present. Despite the abundance of hydrogen and the ubiquity of acid-base reactions, the concentration (or, more properly, the chemical activity) of free hydrogen or hydronium ions is very low in most biological fluids.This is in part because these ions are constantly being consumed in a great variety of metabolic reactions; but the main reason is that there exist in all body fluids relatively large concentrations of buffer substances with basic groups that have a high affinity for protons.It is the presence of these buffers in cells that permits a very rapid metabolic turnover of protons to take place at concentrations as low as 10 -6 or 10 -7 M. Although the concentration of protons is quite low, these ions are highly reactive and their chemical potential (as indicated by the pH) has a pervasive and often critical influence on a wide variety of biochemical systems.Most of the biological effects of pH are due primarily to changes in the ionization of molecules resulting from protonation or deprotonation.By affecting the charge on proteins (enzymes) and other critical reactive groups, pH influences the rates of metabolic reactions, the behavior of biological membranes and membrane transport systems, the binding of molecules, the actions and distribution of drugs and so forth.The mechanisms that serve normally to regulate pH, as well as the whole spectrum of functional disturbances that result from the changes in body fluid pH produced by stress or disease, will probably all find their ultimate explanations in these terms. As an example of the ways in which metabolic pathways can be critically affected by pH, I have chosen to discuss in detail the effects of acid-base changes on glycolysis.In order to put these phenomena in perspective, it will also be necessary to describe more briefly what is known about the effects of pH on certain related processes such as the citric acid cycle, oxidative reactions, gluconeogenesis and the hexose monophosphate shunt.The principles illustrated in this discussion will undoubtedly prove to be applicable to other areas of metabolism as we continue to learn more about the metabolic consequences of acid-base disorders.