1. Loss of NH3 N from faeces and urine between voiding and collection and also losses in expired air, eructed gas and from the skin of wether sheep 2–3 years old given a ration of 400, 700, 1000 or 1300 g dried grass/day have been measured. 2. Losses of NH3 from faeces were negligible, and the loss from urine depended on the temperature and pH at which it was collected. Collection at neutral pH resulted in losses of up to 9.7% of the urinary N as NH3 gas if an acid trap were not incorporated in the collection apparatus. The average loss of N on collection of urine at pH values below 2.0 was 1.33% when the ambient temperature was between 25 and 28°, and 0.97% when it was between 15 and 18°. 3. Very little NH3 was lost in expired air or eructed gas, but when total NH3 loss other than that from urine was determined, an average loss of 3.14±0.55 mg N/kg body-weight per day was found. This loss was independent of food intake and larger than the loss of N in suint which was estimated to range from 0.59 to 2.55 mg N/kg body-weight per day with the rations given. It was concluded that the larger losses found in some experiments were due to splashing of urine on to the fleece during collection and subsequent loss of this urine N as NH3. It was shown that the losses of NH3 N due to errors in urine collection and failure to consider dermal losses ranged between 1.2 and 2.6% of the total determined losses in faeces and urine for sheep fed on a variety of diets and that the losses varied inversely with intake of N.