In the UK, pollution of the water system with nitrate nitrogen leaching from the soil is seen as a major problem and farm animal effluents have been identified as a major source of nitrate pollution. It would, therefore, be beneficial to the livestock producer and to the environment if the nitrogen excretion from animals could be kept to a minimum. To limit the excretion of nitrogen by the pig, it is necessary to supply amino acids in the diet in better agreement with its dietary requirements. This could be achieved either by feeding diets according to the pig's requirements based on age and/or weight (phase feeding) or by improving protein quality. The best protein quality would be that which has the same balance of essential amino acids (EAA) with respect to lysine as that required by the pig, i.e. ideal protein. Diets formulated on the basis of total dietary EAA on an ideal protein basis, using crystalline EAA, could enable lower crude protein (CP) diets to be offered whilst maintaining nitrogen retention (NR). An alternative approach to formulating diets would be to base the formulations on either: 1) currently available, commercial database values for ileal digestible EAA values of ingredients to achieve diets as close to ideal protein as possible but within least cost formulation constraints; or 2) ingredients limited simply to cereals and pulses and supplemented with crystalline EAA to formulate as close to ideal protein as possible. The object of the present experiment was to evaluate diets, formulated on this basis, in terms of nitrogen intake (NI), excretion (NE) and retention (NR) in pigs using balance studies.