The regulation of lactic acid production, the regulation of lactate fermentation and the role of lactate as intermediate in the rumen metabolism was studied. The pH had a pronounced effect on all three processes and therefore buffer capacity of the rumen contents is also described. Starch gave much less rise to lactic acidosis than soluble sugars, as glucose and fructose. Most bacteria grow faster and therefore produce more lactic acid when amino acids and/or soluble proteins are present in the diet. Activity of LDH (lactate dehydrogenase) of mixed rumen microorganisms is regulated by the NADH/NAD(H) balance and the ATP concentration. About 60% of the LDH in mixed rumen microorganisms is fructose-1, 6-diphosphate independent. Megasphaera elsdenii ferments 60 to 80% of the lactate fermented in the rumen of dairy cattle. Lactate accumulates only when the glycolytic flux (hexose units fermented per unit time per microorganism) is high. During adaptation, the glycolytic flux is increased and lactate may accumulate. After adaptation to a certain diet, the number of microorganisms is changed and the glycolytic flux again is normal and lactate is only a minor intermediate in rumen metabolism.