The objective of th~sst udy was to evaluate m i n a l peptide and ammonia nitrogen concentrations and Crude Protein (CP) degradation in Holstein steers fed dets differing in concentrate to alfalfa hay ratios. Four lumen fistulated Holstein steers (21 6+27 kg, body weight) with were used. Animals fed 7 kg of Dly Matter (DM) of dets differing in concentrate (155 g CP kg- of DM; 30% maize, 34% barley, 8% soybean meal, 5% sugar beet pulp, 10% wheat bran, 12% cottonseed meal, 0.3% CaCo3, 0.5% mineral and vitamin premix, 0.2% salt) to alfalfa hay (155 g CP kg- of DM) ratios as 60:40 (C,,:L,,), 70:30 (C,,:L,,), 80:20 (C,,:L,,) and 90:lO (C,,:L,,) in a 4x4 Latin square design (28 days of each period). Steers fed the experimental diets as total mixed ration twice daily at 0800 and 2000 h. Rumlnal fluid was taken, by suction, via m e n fistula on days 24 to 28 of each period. Then, pH and NH3-N, soluble protein-N and peptide-N concentrations were determined. In addition, in situ CP and DM degradation kinetics of various feeds (alfalfa hay, barley grain, soybean meal and fish meal) were determined. Ruminal pH and ammonia-N concentration were significantly affected by the treatments and sampling time @< 0.05). Ruminal pH and ammonia-N concentration decreased from 6.48 (C,,:L,,) and 19.86 (C,,:L,,) to 5.86 (C,,:L,,) and 14.81 (C,,:L,,), respectively, when level of concentrate was increased kom 60 to 90% (p<0.05). Results indcated that the m i n a l peptide-N and soluble protein-N concentrations were not significantly influenced by the dets and sampling time. In steers fed a high c0ncentrate:alfalfa hay ratios (C,,:L,,), m i n a l peptide-N concentration was lower than those fed C,,:L,, and C,,:L,,. In general, m i n a l soluble protein-N was high in samples taken before feedmg, intermedate at 6 h and low at 4 h after feeding. The results showed that the DM and CP degradation parameters of the feeds evaluated in the present experiment are ~nfluencedb y the det composition. It was concluded that the extent of m i n a l CP d

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