The mean response of body protein accretion in growing animals to their amino acid intake is sometimes described by a rectilinear ("broken-line") model and sometimes by a curvilinear model. The response of a population may be curvilinear as a result of averaging individual rectilinear responses or because individual responses are themselves curvilinear. This experiment was undertaken to distinguish these possibilities by examining the responses of individual animals. Eighteen pigs with a mean initial weight of 35 kg and a mean final weight of 73 kg were each fed, in a different sequence, six diets providing a daily nitrogen intake of 0.5, 1.5, 1.83, 2.17, 2.5 or 3.5 g/kg body wt0.75. Each diet was given for 10 d, with complete collections of feces over the last 6 d and of urine over the last 4 d. Rates of nitrogen retention (expressed per kg0.75 per day) were related to rates of nitrogen intake (in the same units) using a rectilinear (broken line), an exponential or a saturation kinetics model. Mean square errors were significantly larger for the rectilinear model than for either of the curvilinear models and were slightly but not significantly less for the saturation kinetics model than for the exponential model.