The metabolism of glycine and serine in chick and pigeon livers was studied with mitochondria and cell homogenates. The avian liver mitochondria actively catalyzed the cleavage of glycine into methylene-THF, 2 CO 2 , and ammonia, but failed to appreciably catalyze CO 2 formation from the α-carbon of glycine. CO 2 formation from the β-carbon of serine was also very small in either the mitochondrial or the homogenate system, whereas the carboxyl carbon of serine was actively decarboxylated. In the avian livers L-serine dehydratase was apparently absent and the activity of 10-formyl-THF: NADP + oxidoreductase was extremely low, if not nil. However, chick liver, but not pigeon liver, was shown to contain an enzyme with the characteristics of D-serine dehydratase, although its exact nature remains to be clarified. The one-carbon compound(s), derived from either glycine or serine in the avian livers was utilized largely for the synthesis of uric acid. With the soluble fraction of avian livers, the yields of 14 C-purine from serine-3- 14 C were about two times larger than those from serine-1- 14 C. When 14 C-glycine alone was employed as the source of both the one-carbon compound and glycine, the yields of 14 C-hypoxanthine, from 14 C-glycine, especially from glycine-2- 14 C, were significantly increased by the addition of mitochondria to the soluble liver fraction, and under these reaction conditions the ratio of the yields of 14 C-hypoxanthine from glycine-1- 14 C and 2- 14 C rose to 1:2.3. It was assumed that the glycine cleavage system in the mitochondria plays an important role as a one-carbon donating system for purine synthesis in the avian liver. Although the avian liver mitochondria could catalyze glycine synthesis as the reverse of the glycine cleavage reaction, the contribution, if any, of the synthetic reaction to purine synthesis seemed to be insignificant.