Urea and wheat straw (3·5:96·5) mixtures with 40% T 1, 50% (T 2), 60% (T 3) and 70% (T 4) moisture were stacked for 9 days. The decline in cellulose and hemicellulose contents during 9 days followed the increase in the moisture level. In another treatment, 4 kg of urea mixed with 100 kg of straw at 45% moisture and 28 days' stacking (T 5) led to maximum loss in cell wall components. The variation in moisture level influenced the microbial activity, as was indicated by temperature changes in T 1 and T 4 stacks. In spite of comparable DMI and apparent nutrient digestibility, both the available digestible nutrients and ME per day were less for T 5 than T 1 straw, indicating that ammonia treatment by means of urea could be better achieved with T 1 than as advocated for the standard T 5 process. The higher OM and cellulose loss with 70% moisture (T 4) did not reduce the availability of digestible nutrients and ME per day from T 4 straw more than those from T 1 straw. For natural fermentation of urea straw, 70% moisture provided the most suitable conditions for improving the nutritive value of straw. In order to check the nutrient losses and growth of undesirable moulds, the dismantling of the stock after the stipulated period is necessary in all cases.