The influence of straight feeds on the fermentation pattern in the bovine rumen was investigated by feeding with hay, grass pellets, dried beet pulp with molasses and rolled oats. The study comprised 3 rumenfistulated dairy heifers, and the parameters measured were ammonia, pH and volatile fatty acids (VFA) in the rumen fluid, and non-glucogenic/glucogenic ratio (NGGR) in the VFA mixture. Typical differences related to the structure and chemical composition of the diets were demonstrated by the parameters measured. Fermentation of the easily digestible feeds resulted in so extensive variations that representative means could be obtained only by analyzing samples collected frequently throughout the feeding cycle. Rumen pH was found highly correlated to total VFA concentration for all the feeds examined, and the regression coefficients showed that diets, based upon beets and oats, were fermented under low buffering capacity.