Rapid incorporation of C14 into circulating albumin occurs following intravenous administration of glycine 2-C14 in man. The maximum C14 activity occurs at about 24 hours, and is followed by a loss of C14 albumin from the plasma which consists of a rapid exponential component with a half-time of about 3 days and a slow exponential component with a half-time of 28 to 39 days. I131 trace iodinated albumin studied concurrently in the same patients leaves the plasma after extravascular equilibration at a rate more than twice as fast as the bio-synthetically C14 labeled albumin, (half-time of 12 to 13 days.). The difference in behavior between I131 and C14 albumin suggests that introduction of I131 into albumin significantly alters the metabolic behavior of the albumin.