For the glycolysis to continue, NAD⁺, wich cells have in limited quantities, must be recycled after its reduction to NADH by glyceraldehyde-3-P dehydrogenase. In the presence of oxygen, the reducing equivalents of NADH are passed into the mitochondria for reoxidation. Under anaerobic conditions, on the other hand, the NAD⁺ is replenished by the reduction of pyruvate in an extension of the glycolytic pathway.

Homolactic fermentation - In muscle, particularly during vigorous activity when the demand of ATP is high and oxygen has been depleted, lactate dehydrogenase catalyzes the oxidation of NADH by pyruvate to yield NAD⁺ and lactate. Much of the lactate produced is exported from the muscle cell and carried by the blood to the liver, where it's reconverted to glucose. The pyruvate can also be exported as alanine by the glucose-alanine cycle.

Alcoholic fermentation - Under anaerobic conditions in yeast, NAD⁺ is regenerated by the conversion of pyruvate to ethanol and CO₂. The acetaldehyde formed by the decarboxylation of pyruvate is reduced to ethanol by NADH.