1. When the rat livers are perfused under the conditions of these experiments with rat blood diluted with saline, the livers remain capable of removing colloidal chromic phosphate normally for 4 hours or more; that is, the reticuloendothelial system continues to function normally.
2. Good rates of bile flow continue, generally for 4 hours.
3. The livers incorporate radioactivity from the amino acids methionine, lysine, and histidine at rapid rates for 1 or 2 hours. Thereafter the rates fall.
4. The specific activity of the free lysine and histidine in the perfusate falls rapidly during the experiments (to 25 or 35 per cent of its original value at 10 minutes).
5. The fall in rate of incorporation of radioactivity is attributable to the fall in amino acid specific activity.
6. Addition of a complete amino acid mixture to the perfusate does not appear to have any stimulatory effect on incorporation of radioactivity from labelled amino acids.
7. With lysine, on the assumption that incorporation is due to new protein formation, there is a rate of synthesis equivalent to 230 mg. of plasma protein per 100 gm. of rat per day. This result is in agreement with turnover data obtained from rats in vivo.
8. The results emphasize once again the importance of precursor specific activity in the interpretation of metabolic experiments with labelled amino acids.