An experiment was carried out to examine the effect of rumen protozoa on feed utilization, nitrogen balance and urinary excretion of purine derivatives (PD) in faunated and defaunated lambs fed a hay and concentrate diets. There was no change in digestibilities of dry matter (DM) and organic matter (OM) with or without protozoa, however the digestibilities of crude protein and crude fibre tended to decrease in defaunated lambs, but not significantly (P>0.05). Defaunation decreased nitrogen (N) balance and urinary N excretion. Consequently, retained N was higher (P<0.05) in defaunated than in faunated lambs. Mean plasma allantoin concentration was higher (P<0.05) in defaunated (41.2 μmol/l) than in faunated lambs (25.5 μmol/l). Total urinary PD excretion and allantoin excretion were higher (P<0.05) in the defaunated than in the faunated lambs. From the above results, it can be concluded that rumen protozoa have an important role in N utilization, plasma allantoin levels and total PD excretion in lambs.