Hypothalamic tryptophan (TP), serotonin (5HT) and 5-hydroxy-indoleacetic acid (5HIAA), serum tryptophan (free and total), and circulating corticosterone and aldosterone were measured over 24 hr in 60-day-old male rats fed high (60%) and low (6%) protein diets. The diets were administered during specific stages of rat growth: fetal phase and suckling (when the process of mitosis is most rapid), post-lactation, and from the fetal phase through to the adult stage. In all groups, there was a direct correlation between protein intake and hypothalamic and serum TP levels. Remarkable decreases in the amplitude and mesor of the aldosterone rhythm were observed when the hyperprotein diet was administered in the fetal and suckling phases, or from the fetal phase until the adult stages. The rhythm of the serotonin system was significantly altered by a low protein diet: decreased 5-HT amplitude (low protein diet in the fetal stage and suckling), decreased 5-HT amplitude and deranged 5-HIAA rhythm (low protein in the post-lactation period), deranged 5-HIAA rhythm (low protein diet throughout the experiment). Lastly, there was a remarkable increase in the corticosterone mesor in rats administered a low protein diet after weaning and in rats constantly given this diet. Growth was greatly inhibited in these two groups of animals, therefore, it cannot be excluded that the high levels of circulating hormone resulted from their debilitated condition.