An in vitro study was devised to evaluate the fermentation pattern of three commercial rations, two high fiber (BS and PR) and one high in concentrate (CM) and compare them with alfalfa (AL) and barley (CB) as fibrous and energetic feed ingredients, respectively. After the pre-digestion process of the substrates, three in vitro incubation series were performed using faecal material of 4 horses as inoculum. Gas pressures were measured from the bottles at intervals of 2 up to 24 hours, and then bottles were opened to determine pH, VFAs and NH3 content. CB produced greater amount of gas respect to other diets studied where BS produced the lowest levels of gas. The production of VFAs was also higher in CB and no differences were seen between AL, PR and CM rations. Both CB and CM enhanced production of propionate, whereas PR, AL and BS tended to produce more acetate. Although both PR and uction, NH levels were higher in the latter one, suggests more efficiency in energy: protein synchronization for the PR ration.