Analysis of evidence of associations among dietary protein content, brain amino acid and serotonin concentrations, and protein self-selection by rats suggests that 1) protein intake is not regulated precisely, although rats will select between low and high protein diets to obtain an adequate, but not excessive, amount of protein; 2) associations between brain serotonin concentration and protein intake are weak, although consumption of single meals of protein-deficient diets will elevate brain serotonin concentration; 3) the nature of signals that drive rats to avoid diets containing inadequate or excessive amounts of protein remains obscure; (4) whole brain amino acid and serotonin concentrations are quite stable over the usual range of protein intakes, owing to competition among amino acids for uptake across the blood-brain barrier and effective metabolic regulation of blood amino acid concentrations; 5) protein intake and preference are not in themselves regulated, but what appears to be regulation of intake and preference is a reflection of the responses of systems for control of plasma amino acid concentrations; and (6) the relative stability of the average protein intake of groups of self-selecting rats (which gives the appearance of regulation) results from averaging the variable behavioral responses--learned aversions and preferences--of rats to the variety of sensory cues arising from diets that differ in protein content.