In order to describe the response of broiler breeder hens to dietary amino acids and to develop an effective model for the precise feeding of these birds after sexual maturity, accurate estimates of the amounts of each amino acid required for maintenance and egg production are needed. The maintenance requirement for threonine and lysine were estimated in two different experiments by measuring the nitrogen balance of adult male cockerels. Measured amounts of a diet first-limiting in threonine or lysine were fed by intubation each day for 4 d to give a range of intakes (unbalanced series) of from 0 to 239 mg threonine/kg or 0 to 40 mg lysine/kg body weight. To confirm that threonine or lysine was first-limiting and that the response obtained was to threonine or lysine and not to protein, a second series of diets was used (balanced series) in which synthetic threonine or lysine was added to each diet in the unbalanced series. A nitrogen-free diet containing energy, vitamins and minerals was made available ad libitum during the balance period, to ensure that the birds remained in positive energy balance. The balance period was three days and excreta were collected in colostomy bags (threonine) or in trays (lysine) during that time. The nitrogen content of the excreta was determined on dried homogenised samples. The resultant linear regression equations describing the effect of threonine or lysine intakes on nitrogen retention were: N retention = -230.4 (± 27.6) + 4.134 (± 0.274) I (R2 = 91.9) and N retention = -256.8 (± 37.7) + 6.89 (± 1.37) I (R2 = 70.8), respectively, where I is the intake of threonine or lysine in mg/kg body weight day. The threonine and lysine required to maintain the body at zero nitrogen retention was therefore estimated to be 56 and 37 mg/kg body weight day, respectively.