Lactic acid fermentation is a form of anaerobic respiration that occurs in animal cells in the absence of oxygen. Glycolysis occurs normally, producing 2 molecules of ATP, 2 molecules of NADH and 2 molecules of pyruvate but the pyruvate is not metabolised to CO₂ in the citric acid cycle. Furthermore, the electron transport chain (which uses O₂) does not function. If the ATP needs of a cell outpace oxygen supply (such as during strenuous exercise), cells can only use fermentation for ATP production. Although it is a dead-end, the conversion of pyruvate to lactate regenerates NAD⁺ and this allows glycolysis to continue in the muscle cell. Lactate diffuses out of the cell and into the blood.

Certain cells, such as cardiac muscle cells, are highly permeable to lactate. Lactate is converted into pyruvate and metabolised normally (ie: via the citric acid cycle). Since these cells are highly oxygenated, it is unlikely that lactate would accumulate (as is the case in oxygen-starved muscle cells). This also allows circulating glucose to be available to muscle cells.

Any excess lactate is taken up by the liver, converted to pyruvate and then to glucose. This, along with the production of lactate from glucose in muscle cells constitutes the Cori cycle.

Phosphofructokinase (PFK) is inhibited by a low pH and this prevents the formation of excess lactate and/or lactic acidosis (sudden drop in blood pH). PFK catalyses the first irreversible step in glycolysis.

Muscle cramps have been associated with levels of lactic acid in the blood, especially as a result of electrolyte imbalance caused by sweating. However, certain activites, which do not induce sweating or seem strenuous, still produce cramps (such as writing or playing an instrument) so it is unlikely that lactic acid is anything more than a minor factor.