The present experiment was carried out in order to investigate the digestion and utilization of dietary nitrogen in sheep fed on a low-quality hay and orally supplemented with casein at various levels, and the results obtained were as follows ; 1). The digestibility of crude protein was fairly increased with an increase of casein added orally. The digestibilities of organic matter, crude fibre and nitrogen free-extract were not changed with an oral casein supplement. 2). Urinary nitrogen excretion was fairly increased with anincrease of oral casein addition, and the retained nitrogen was not increase with an increased of oral casein supplements. It appears that the nitrogen from added casein was mostly excreted into urine. 3) . Ruminal ammonia level was clearly increased with an increase of oral casein supplements, and subsequently, ruminal pH tended to be higher after feeding hay with casein than after feeding hay alone. The level of ruminal VFAs tended to be lower in feeding hay with casein than in feeding hay only. 4). The concentration of blood urea-nitrogen increased gradually with an increase of casein added. The level of plasma total protein was not changed by the change of amounts of supplemented casein.