In suprachiasmatic nucleus (SCN)-lesioned and blind rats the percentage of protein intake would be decreased, while that of carbohydrate intake increased as a result of the low level of glucagon. Leibowitz et al. reported that macronutrient selection of rats showed the daily rhythm under a light-dark cycle. Serotonin might be involved at a specific time of day in the regulation of macronutrient selection. The daily change in macronutrient selection also suggests an involvement of the SCN. It has been repeatedly emphasized that SCN neurons receiving retinal input are involved in the regulation of energy metabolism. Furthermore, signals from SCN neurons are transmitted to peripheral organs, such as the liver and adrenal glands, through the autonomic nervous system. The hereditary microphthalmic rats which have fewer cells and presumably also reduced functional activity in the SCN become obese at 8 weeks.