The tracers L 15N-proline and L(1-13C)-leucine were used to explore the synthesis of skin proteins in vivo in rabbits. They orally received a single dose containing an equimolecular mixture of L(1-13C)-leucine and L15N-proline. The changes in the amounts of these tracers in blood and skin were monitored for a total of 8h. The data showed the appearance of the two tracers in blood within 15min and their clearance in 8h. They were both rapidly (15min) incorporated into skin proteins, but more proline was incorporated than leucine. We therefore consider L 15N -proline to be a better tracer than ...