Four sheep were fed twice daily a diet of 67% ryegrass hay and 33% concentrate during three, 4-wk periods. The diet was supplemented with one of three N sources: urea, casein or ovalbumin. Urea was the only supplemental N source during Period 1. Sheep were fed either casein or ovalbumin during Period 2, followed by switching of the supplements during Period 3. Statistical comparisons were made only on data obtained during Periods 2 and 3. Ruminal concentrations of NH3, free amino acids and peptides were measured over an 8-h period after feeding. The peptide assay was based on the enhanced fhaorescence of peptides, relative to amino acids, obtained from reaction with fluorescamine at pH 6.2. Ammonia accumulated rapidly to high concentrations (maximum 38 mM at 1 h) after feeding Urea. Ammonia was intermediate with casein and greater (P < .05) than with ovalbumin. Free amino acids were greater (P < .05) with casein (maximum 1.4 mM at 1 h) than with oval- bumin. Free amino acids were intermediate with feeding of urea. Only transient accumulation of peptides occurred when casein was fed: ruminal peptides increased to 3.8 mM at 1 h, but declined to prefeeding levels (.2 mM) by 3 h. Peptides in ruminal fluid from sheep fed ovalbumin were lower (P < .01) and (similar to urea)did not change from prefeeding levels (.2 to .3 raM) throughout the sampling period. It was concluded that slowly degraded proteins, such as ovalbumin, will not give rise to significant peptide levels, whereas rapidly degraded proteins, such as casein, will yield substantial levels of peptides during ruminal protein degradation.