Current waste management systems for cattle feedlots and swine facilities result in nitrogen losses of approx. 75%. Most of this loss occurs through the rapid hydrolysis of urinary nitrogen (urea) to ammonia, which volatilizes into the atmosphere. This contributes to odor; environmental problems, and loss of a valuable fertilizer resource. Urease inhibitors cyclohexylphosphoric triamide (CHPT) and phenyl phosphorodiamidate (PPDA) were used to control the hydrolysis of urea in duplicate one-liter slurries of cattle and swine wastes (1:1 g:g feces to urine). With cattle waste (3.3 g urea l(-1)) and swine waste (4.8 g urea l(-1)), both inhibitors at 10 mg l(-1) of waste prevented hydrolysis of the urea for 4-11 days, and then a gradual hydrolysis occurred until complete at day 28. Hydrolysis of urea in untreated cattle or swine waste (controls) was complete within one day. Addition of the inhibitors once per week was the most effective method of preventing urea hydrolysis. Weekly additions of 10, 40 or 100 mg of PPDA per liter of cattle waste (5.6 g urea l(-1)) prevented 38, 48 and 70% of the urea from being hydrolyzed after 28 days, respectively. With swine waste (2.5 g urea l(-1)) these PPDA concentrations prevented 72, 92 and 92% of the urea from being hydrolyzed after 28 days, respectively. PPDA additions were stopped after 49 days, and all urea at the three PPDA concentrations was hydrolyzed after 70 and 84 days for the cattle and swine wastes, respectively. These results offer a strategy for significant control of ammonia emissions from livestock facilities and increasing the fertilizer value of wastes by improving the nitrogen to phosphorous ratio for plant growth.