The enzyme polyphenol oxidase (PPO) may be responsible for increases in both dietary-nitrogen utilisation and the concentration of polyunsaturated fatty acids in ruminant products when animals eat red clover (Lee et al. 2004). The enzymatic reaction requires the presence of oxygen and therefore is assumed to be inhibited in the anaerobic rumen. However, little information exists as to the concentration of oxygen in boluses and whether this could sustain the aerobic enzymes activity in the reticulo-rumen. This study investigated the concentration of oxygen in grass-boluses measured in vitro or in vivo in the rumen of two rumen fistulated cows.