In the course of a general study of the chemical changes taking place in the manure heap, a number of experiments were made with the object of determining the loss of nitrogen during the aerobic fermentation of urine, straw and faeces, both separately and in various combinations. In the very first experiment with faeces, however, instead of finding a loss of nitrogen, appreciable gains were recorded, and this opened the enquiry which forms the subject of the present paper. In all the experiments a current of washed air was drawn for several hours daily through the flasks containing the substance under examination, any ammonia volatilized being retained by N/10 sulphuric acid. At the end of the experiment the total nitrogen left in the fermented substance, added to any nitrogen recovered in the wash flasks, was compared with the original total nitrogen.