The rumen of dairy cows is a place of complicated digestion processes that increase the total acidity. If the acid contents in the rumen exceeds a specific limit (often set at pH 5.8), so called acidosis which may occur and negatively affect the feed intake, the production and quality of milk and the general health condition of the animals, including limb diseases (laminitis). The risk of acidosis can be reduced by higher proportion of physically efficient neutral detergent fibre (peNDF). The aim of the study was using special probes rumen eCOW describe the process of pH in the rumen of dairy cows fed with different feed rations on the border of the risk of acidosis. Measurements showed large differences in the pH in the rumen during the day as the cows and between periods. Dairy cows respond to many influences, about which we can only assume, but they are capable the fluctuations of pH fairly quickly eliminated.