Free amino acids were measured in extracts from liver, kidney, and skeletal muscle (m. psoas major) of fetal, newborn, and adult rabbits. The total amount of free amino acids is at a higher level in fetal tissues than in adults; this difference is rather small in the liver, slightly larger in renal and very marked in muscle tissue. Perinatal changes of individual amino acid concentrations are different in each tissue studied. Many of such changes can be interpreted as a reflection of the metabolic sitation in each organ, e.g., the great need for essential amino acids during phases of very active protein synthesis. There were essentially no good correlations between developmental changes occurring in tissues and those in plasma.