Urease inhibitors have been successfully used to reduce ammonia (NH₃) emission from urea-based fertilizers. However, studies on its effectiveness with manures have produced inconclusive results. Field and greenhouse studies were conducted to investigate the effectiveness of different rates of urease [N-(n-butyl) thiophosphoric triamide; NBPT] with and without nitrification inhibitor (NI) in reducing NH₃ emission from surface-applied liquid pig manure (LPM) and solid beef manure (SBM). Ammonia emission was measured with acid-charged discs at seven dates for 28 d. Total NH₃ emission (% of applied N) ranged from 4.3% to 8.2% in untreated LPM and 8.2% in untreated SBM. The corresponding NH₃ emission was 6.8%–7.4% in LPM treated with NBPT, 5.0%–12.3% in LPM treated with NBPT + NI (double inhibitor; DI), and 6.0%–10.8% in SBM treated with DI. In the field study, NH₃ emission was not significantly different between either LPM or SBM treated with and without DI. In the greenhouse, NBPT did not significantly reduce NH₃ emission from LPM, whereas DI applied at a lower rate significantly increased NH₃ emission from LPM. In conclusion, addition of NBPT to manure did not have any significant environmental benefit, whereas a combination of NBPT and NI increased NH₃ emission from manure.