In delayed-flood rice culture, it is recommended urea be the N source for the large, early N application and it be applied immediately before establishment of the permanent flood.  Urea has many fine qualities, but it also has an undesirable characteristic of being prone to ammonia volatilization losses if not soil incorporated within a couple of days after surface application.  Most commercial rice fields require 5 to 10 days to get the floodwater across the field and thus, there is potential for substantial ammonia volatilization losses of urea applied preflood.  Urease inhibitors have been promoted as a means to significantly slow ammonia volatilization losses from urea.  Two products that can be coated on urea and are being promoted as containing urease inhibitors are Agrotain and Nutrisphere.  Laboratory and field studies were conducted comparing the ammonia volatilization and influence on rice grain yield of urea, ammonium sulfate, Agrotain coated urea or Nutrisphere coated urea applied at different times prior to flooding.  Ammonium sulfate followed by Agrotain coated urea volatilized ammonia the least of the four N fertilizers.  Urea and Nutrisphere coated urea volatilized ammonia similarly and in much greater amounts compared to ammonium sulfate and Agrotain coated urea.  Nutrisphere did not have any inhibitory effect on ammonia volatilization of urea.  When the N fertilizers were applied 5 and 10 days prior to flooding, rice yields were the largest when ammonium sulfate and Agrotain treated urea were the N sources.  Rice yields were lower and no different between Nutrisphere coated urea and uncoated urea when applied 5 and 10 days prior to flooding.  Thus, Agrotain treated urea or ammonium sulfate should be used as the preflood N source if greater than 3 days are required to establish a flood.