NH3 gaseous losses from urea (surface applied or incorporated to 2.5 to 3 cm depth in soil at 30-60% of maximum water holding capacity and at 20 deg C) and the effect of N-Serve on the transformation rate of urea, ammonium sulphate and two samples of IBDU were studied. Over a period of 28 days, the NH3 gaseous losses varied from 0 to 18.7% of the nitrogen content of the fertilizer. Zero or lowest losses were observed with the slow-acting fertilizers. The highest losses were observed when urea was left on the surface of soil at 30% of maximum water capacity, particularly on soil of light mechanical composition, higher pH and higher degree of base saturation. It was particularly in this type of soil that N-Serve increased the NH3 losses from urea, due to accumulation of NH3-N in the soil and to increased pH.