Since the concept of movement of urea nitrogen across the rumen wall was proposed by Simonnet, Le Bars, and Molle (1957), it has gained wide acceptance. Experiments using isolated, washed, saline-filled rumens of anaesthetized sheep (Houpt 1959; Hogan 1961; Ash and Dobson 1963; Packett and Groves 1965) and rumen pouches (Juhasz 1965; Houpt and Houpt 1968) have shown that urea can cross the rumen wall, appearing in the lumen mainly as ammonia. Gray, Pilgrim, and Weller (1958), Harris and Phillipson (1962), and Kay and Phillipson (1962) have shown that, with diets low in protein, more nitrogen may pass from the abomasum than is contained in the feed, indicating entry of endogenous nitrogen to the rumen. As the salivary urea excretion rate may account for only about 0�5 g nitrogen per day (McDonald 1948), it has been implied that the gain in nitrogen results from passage of urea nitrogen through the rumen epithelium from the blood (Houpt 1959; Kay and Hobson 1963; Juhasz 1965; Houpt and Houpt 1968; Tillman and Sidhu 1969).