Brazil has arid regions where livestock production is limited by forage source. However, some native herbaceous browses have a dry tolerance and have been used as animal feeds. Some of those plants have anti nutritional compounds such as tannins that can interfere on intake and digestibility. Tannins have a high affinity to proteins and could make these molecules unavailable for animal. Compounds as polyethylene glycol (PEG) has been used on tannin studies, because it has more affinity with tannins than proteins. Based on that, it is possible to evaluate the nutritive potential of tanniniferous plants, using PEG in gas based techniques for assessing anti nutritional factors in tanniniferous plants for ruminants. The aim of this work was to investigate the biological activity of tannins using the in vitro gas method with the addition of polyethylene glycol (PEG).