Six diets based on maize and soya-bean meal were prepared. The basal diet had a crude protein (CP) concentration of 171 g/kg. The same ingredients, together with maize starch, were used to give a diet with CP 131 g/kg. This diet was supplemented with all (positive control) or with three of the four amino acids lysine, threonine, tryptophan and methionine. Each diet was given at two rates of intake. From the results, the optimum ratio between these four amino acids at different planes of intake was calculated. Amino acid digestibility in the basal diet was measured at three intakes, using pigs fitted with T cannulas at the terminal ileum.The results showed that the optimum ratios between the four amino acids (expressed in terms of digestible amino acids) were not affected by the rate of food intake. The optimum ratio between the four amino acids (i.e. when they are equally limiting) was lysine 1-00, methionine + cystine 0-61, threonine 0-64 and tryptophan 0-20. Digestibility was not affected by the level of intake.