Two lactating Merino ewes with a rumen fistula were housed in metabolic boxes for N-balancc and fed a diet be composed of oats, sugar beet pulp and hay. After adaptation on dietary ammonium sulfate suplementation each of the ewe daily received 21 g15N-ammonium sulfate during tlie 10-d full time period. The daily intake of dry matter was 1.9 or 1.5 kg, of energy unit for cattle 1074 or 864 and of crude protein 266 or 187 g by ewe 1 or 2 respectively. The utilization of NPN was 55 or 59 per cent for synthesis of whole protein, 11 or 4 per cent for synthesis of milk protein and 22 or 43 per cent for N-retention. The utilization of true protein from feedstuffs was 62 or 71 per cent for synthesis of whole protein, 12 or 5 per cent for synthesis of milk protein and 25 or 51 per cent for N-retention by ewe 1 or 2 respectively.