A laboratory study was conducted to evaluate how rate and timing of urease inhibitor application affects ammonia emissions from simulated beef cattle feedyard surfaces. The urease inhibitor N-(n-butyl)thiophosphoric triamide (NBPT) was applied at rates of 0, 1 and 2 kg/ha, at 8, 16, and 32 day frequencies, and with or without simulated rainfall. Synthetic urine was added every 2 days to the manure surface. Gaseous NH3 was trapped in a sulfuric acid solution using a vacuum system and analyzed for nitrogen using automated procedures. NBPT applied every 8 days was most effective, with the 1 and 2 kg/ha NBPT treatments resulted in 49 and 66% reduction in NH3 emission rates, respectively. The 8-day, 1 kg/ha NBPT treatment had the most promising cost/benefit ratio of 0.48. Simulated rainfall reduced the NH3 emission rates slightly as compared to the non-rainfall treatments, though the differences were not statistically different.