The effect of temperature, moisture level and treatment period on the ammoniation of wheat straw by 75 9 urea per kg wheat straw was investigated in a 4 x 2 x 6 factorial experiment. Independent variables included temperatures of 4; 14; 24 and 35°C, moisture levels of 250 and 375 g/kg wheat straw and treatment periods of 0; 1; 2; 4; 6 and 8 weeks. Dependent variables investigated were total nitrogen content, urea content, free ammonia nitrogen content and in vitro organic matter digestibility (IVOMD). Temperature, moisture level and treatment period significantly (P ≤ 0,01) affected ammonia release from urea. The efficiency of ammoniation was accordingly affected. Slow ammonia release at 4 °C (at both moisture levels) and at 14 °C (only at the lower moisture level) resulted in lesser improvements in IVOMD after a treatment period of eight weeks. Ammoniation was accelerated at higher temperatures, particularly at the higher moisture level. Comparable IVOMD values were obtained after a treatment period of one to two weeks at 35 °C and approximately six weeks at 24 °C. Less ammonia was released from urea at 35 °C and the moisture level of 375 g/kg, possibly owing to a decrease in urease activity at this temperature. Significant (P ≤ 0,01) temperature x treatment period and moisture level x treatment period interactions were obtained, indicating that the slow reaction at lower temperatures and at lower moisture levels can partly be compensated for by a longer treatment period.