The effect of alteration of light-darkness cycle on the protein metabolism was studied in the rat. The light-darkness cycle was altered either every 3 or 9 days, and animals consumed diets containing 8 or 25% casein. The results were summarized as follows : 1) Food consumptions and body weight gains of the 25% casein groups were higher than those of the 8% casein groups, and, among the animals consumed 25% casein diet, the light-darkness cycle altered group had lower food consumption and body weight gain than the unaltered group. 2) Weights of liver and adrenal gland per l00g body weight were not different with the dietary protein levels, but, at the end of experimental period, the 8% casein diet group of which light-darkness cycle altered every 9 days had the smallest liver weight and the largest adrenal gland weight. 3) Liver nitrogen and plasma protein concentrations of the 25% casein groups were slightly higher than those of the 8% casein groups. 4) Percentages of nitrogen retention of the 25% casein groups at period III were slightly lower in the light-darknerr cycle altered animals than that of the unaltered group.