The influence of starvation on incorporation of amino acid mixture-$^{14}C$ into protein and lipid was studied in rats weighing 200~220 g. Rats were divided into two groups : control group was fed ad libitum, and starved group fasted for 24 hr. All animals were administered intraperitoneally with a single dose of 4 ${mu}Ci$ of amino acid mixture-$^{14}C$ mixed with 20 mg of L-alanine per 100 g body weight in 0.5 ml of water. After three hours, rats were killed and 0.2 g of each tissue from brain, heart, lung, liver, spleen, kidney, stomach, small and large intestine, and femur muscle respectively were excised for the determination of specific radioactivity of protein and lipid of each tissue. The rates of amino acid mixture-$^{14}C$ incorporation into protein and lipid differed with organs. The rate of protein synthesis was higher in order of intestine, stomach, kidney, spleen, liver, lung, and the lowest in muscle, but that of lipid synthesis was higher in order of lung, kidney, spleen, and heart in fed rats. The ratio of specific radioactivity of protein to that of lipid in fed group was approximately two, but the ratio was decreased to 1.2 by the starvation, reflecting that amino acids are more utilizable for the synthesis of protein than that of lipid in fed state. The rate of amino acid-$^{14}C$ incorporation into lipid elevated by about two folds in fasted group compared with fed group, but the rate of protein synthesis was affected slightly by the starvation, suggesting that the utilization of amino acids for lipid synthesis is regulated by nutritional condition.