A nitrogen budget procedure was used to estimate the loss of manure N from three swine barns with liquid manure systems. The loss of N was determined as the difference between animal plus feed N inputs and animal plus manure N outputs. Estimates of ammonia loss were also obtained directly from measurements of flow rates and ammonia concentrations of ventilated air in two barns. Between 5% and 27% of the excreted N was unaccounted for or lost as ammonia after extensive monitoring periods of 415 to 617 days. There was good agreement between the budget and direct methods for estimating N loss in one barn. Reasons for a discrepancy in estimates in a second barn are discussed. The extent of N loss was generally related to the manure management system in the barn. The greater the exposure of fresh manure during collection or storage, the greater the N loss. Temperature had both direct and indirect effects (through increased ventilation rates) on ammonia losses from barns.