<jats:p> <jats:list list-type="explicit-label"> <jats:list-item><jats:p>The uptake and incorporation <jats:italic>in vitro</jats:italic> of radioactive leucine and lysine in rat pancreas fragments have been studied.</jats:p></jats:list-item> <jats:list-item><jats:p>The uptake and incorporation of both leucine and lysine were found to be influenced by the concentration of leucine in the medium.</jats:p></jats:list-item> <jats:list-item><jats:p>Extracellular and intracellular free amino acids can be distinguished by washing the tissue fragments at 0 °C.</jats:p></jats:list-item> <jats:list-item><jats:p>Though it takes some 20 min for each of both radioactive amino acids to reach maximum concentration in the intracellular soluble amino acid pool, their incorporation into protein becomes linear shortly after the start of incubation in a medium with labelled amino acids.</jats:p></jats:list-item> <jats:list-item><jats:p>In chase‐type experiments the incorporation of each of both radioactive amino acids stops immediately when the labelled medium is replaced by unlabelled medium, though the free, intracellular amino‐acid radioactivity declines only slowly.</jats:p></jats:list-item> <jats:list-item><jats:p>These results indicate that in the pancreas of the fasting rat <jats:italic>in vitro</jats:italic> the amino acid molecules incorporated into protein are derived from the cxtracellular pool. Other molecules enter and leave the intracellular amino acid pool but do not contribute to incorporation.</jats:p></jats:list-item> </jats:list> </jats:p>