The effects of several environmental factors on the volatilization of ammonia from surface applications of urea on Dickson silt loam soil were studied under laboratory conditions. Increasing temperature and/or soil pH markedly increased ammonia volatilization. Ammonia volatilization from pH 6.5 soil was essentially the same when urea was topdressed, mixed with the top ¼ inch of soil or topdressed and watered into the soil with ¼ inch of water. Thus, it may be assumed that urea diffused through the soil to approximately the same extent with each of the three treatments. Some ammonia volatilization occurred without a concurrent drying process in the soil, but greater volatilization occurred when moisture was lost from the soil. The drying rate, however, varied over a wide range without affecting the rate of ammonia volatilization. Under conditions of very rapid drying, the soil became dry after 4 to 5 days of aeration, and ammonia volatilization was markedly decreased, presumably because hydrolysis of the urea was retarded due to lack of moisture. Ammonia volatilization was found to be directly related to initial soil moisture content presumably through the effect of this variable on the duration of the drying process.