The use of slow release nitrogen fertilizer (SRNF) could contribute to improve the efficiency of rice production. A field study was established to evaluate the efficiency of the use of a SRNF. Six treatments were compared, three formulations of SRNF (50 % SRNF + 50 % urea, SRNF 60 % + 40 % urea y 70 % SRNF + 30 % urea), two conventional fertilizers (urea y sulfur coated urea) and a control. All treatments were replicated three times and were managed according to the management plan of the farm where the experiment was conducted. Number of tillers, total rice grain yield and biomass N contents were used as comparison variables among treatments. Total rice grain yields, as well as the other variables, showed no significant differences among treatments. The lack of response to N fertilization was attributed to high levels of N in the soil which were present in sufficient amounts to produce optimum rice yields. As a result the efficiency of the use of SRNF could not be evaluated.