Relationship among food composition, plasma and brain tryptophan and 5-hydroxytryptamine (5-HT) were studied by measuring both carcadian feeding and meal responses in self-selecting rats. Under ad libitum feeding conditions in which young rats were given a choice between two isocaloric diets (0 and 60% casein) and groups were killed at 3-hour intervals throughout the day, several circadian rhythms become apparent. Protein and carbohydrate energy, the plasma tryptophan:neutral amino acid ratio (TRP:NAA) and brain TRP, but not 5-HT concentrations showed statistically significant circadian variations. During the dark hours of plasma TRP:NAA ratio and brain TRP concentrations were inversely associated with the protein concentration of meals previously selected. In meal response studies, rats were similarly given a choice of diets, but were killed 20 minutes after their selected meals. Plasma TRP:NAA ratios, brain TRP and 5-hydroxyindole levels were inversely correlated with the protein concentration of the meals selected. Absolute quantities of protein, but not carbohydrate, fat or total energy, were inversely related to the plasma TRP:NAA ratio and brain TRP concentration. It is concluded that the circadian changes in the plasma TRP:NAA ratio and in brain TRP amd 5-hydroxyindole concentrations are under the continual influences of food choices made during spontaneous feeding activity.