The effect of oral administrations of nicotinohydroxamic acid (NHA) and neomycin (NM) in ammonia and urea metabolism were investigated in the rat. NHA and NM lowered the blood ammonia level, and NHA increased urinary excretion of urea and depressed urease activity in the stomach and the colon, while NM had a smaller effect on urinary excretion of urea but depressed urease activity in the colon.