Daily weight gains and food intakes were measured in male, 120-g rats fed graded levels of dietary casein. After 14 d, serum and brain amino acid concentrations were measured. All physiological responses were tested for a functional relationship to dietary casein concentration. Food intake, weight gain and many serum amino acid profiles were shown to be saturable functions of percent casein in the diet. In general, essential amino acids increased in serum with increasing dietary casein concentration while nonessential amino acids decreased with increasing dietary casein concentration. Brain amino acid concentrations were shown to be linear functions of serum levels with the exceptions of phenylalanine and the acidic amino acids. Most amino acids showed a smaller range of values in brain than in serum. The exceptions were the levels of threonine, glutamine, serine and histidine, which were three times greater in brain than in serum. Brain levels of the neutral amino acids tryptophan and tyrosine were highly correlated with the amino acid/neutral amino acid ratios in serum, whereas leucine was negatively correlated. Brain histidine, which was inversely correlated with dietary casein, was found to correlate with specific food intake patterns. The four-parameter mathematical model for physiological responses was able to predict all the observed saturation type responses in the experiment.