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Transition is defined as the period from 3 weeks before parturition until 3 weeks postpartum (Grummer 1995). This period is characterized by changes in endocrine status and by major alterations in the requirement of nutrients, vitamins and minerals. Energy requirement, in particular, is increased from some 60 MJ NEL/d during late pregnancy to 150 MJ NEL/d (milk yield 35 kg/d). This numerical consideration disregards the decrease of voluntary food intake before and mainly at the day of parturition and the delayed dry matter intake after parturition, which may exacerbate the difference in energy intake ante- or postpartum. At parturition, the dry period diet is replaced by an ad libitum offered diet which causes changes of rumen fermentation due to the increase of dry mater intake and the higher energy density of the milk production diet. Hence, fermentation parameters in the rumen such as pH, VFA concentrations and osmotic pressure are changed, which influence transport of ions and nutrients across the rumen epithelium. The current knowledge about these diet- dependent effects on transport properties of the rumen epithelium are summarized, possible limits of adaptation will be discussed, and perspectives of future research are proposed.