Studies indicate that hypothalamic monoamine systems involved in the control of food intake have specific effects on temporal feeding patterns and on appetite for specific macronutrients. Based on the evidence obtained in rats, it is proposed that serotonin acts, in part, through a satiety mechanism of the medial hypothalamus, to reduce ingestion of carbohydrate while sparing protein intake. In controlling the ratio of carbohydrate to protein intake, this serotonergic system, which is responsive to the anorectic agent fenfluramine, is believed to function in direct opposition to the alpha 2-noradrenergic system of the paraventricular nucleus, which inhibits satiety for carbohydrate and thereby potentiates the size of carbohydrate meals. This serotonergic system may also indirectly oppose the catecholaminergic systems of the lateral hypothalamus, which mediate amphetamine anorexia and which inhibit a hunger-stimulating system for protein intake, thereby delaying the initiation of protein meals. Examination of the rats' normal eating patterns, in conjunction with particular biochemical analyses, has indicated specific points in the circadian eating cycle where these hypothalamic monoamine systems, in association with changes in circulating hormones and nutrients, may be physiologically activated.