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Rumen adaptation plays an important role in the productive cycle of dairy cattle. In this study, the time course of functional rumen epithelium adaptation after a change from hay feeding (ad libitum) to a mixed hay/concentrate diet was monitored by measuring Na+ transport rates in Ussing chamber experiments. A total of 18 sheep were subjected to different periods of mixed hay/concentrate feeding ranging from 0 weeks (control; hay ad libitum) to 12 weeks (800 g hay plus 800 g concentrate per day in two equal portions). For each animal, the net absorption of sodium was measured following the mixed hay/concentrate feeding period. Net Na transport, Jnet, significantly rose from 2.15 0.43 (control) to 3.73 1.02 eqcm-2h-1 after one week of mixed hay/concentrate diet, reached peak levels of 4.55 0.50 eqcm-2h-1 after four weeks and levelled out at 3.92 0.36 eqcm-2h-1 after twelve weeks of mixed feeding. Thus, 75% of functional adaptation occurred during the first week after diet change. This is in apparent contrast to findings that morphological adaptation takes approximately 6 weeks to reach peak levels. Hence, early functional adaptation to a mixed hay/concentrate diet is characterized by enhanced Na absorption rates per epithelial cell. Absorption rates are likely to be further enhanced by proliferative effects on the rumen epithelium (number and size of papillae) when concentrate diets are fed over longer periods of time. Early functional adaptation without surface area enlargement of the rumen epithelium appears to be the first step in coping with altered fermentation rates following diet change.