The ruminal physicochemistry is the result of the biocenosis activity in interaction with host (absorption of fermentation terminal products through the ruminal wall, motricity and transit, etc). It characterizes the biotope and also influences the microbiota. In young ruminant the establishment of the microbiota is gradual and relatively quick (Fonty et al., 1987). The ruminal fermentation and degradation are showed variable with the age (Rotger et al., 2008). Are the ruminal parameters time-stable in the adult cow? If variability is observed, is this variability linked to the diet composition? This study addresses these questions.