Abstract
Tritiated thymidine autoradiographs prepared from sections of guinea-pig thymus were compared with sections from animals treated with the mitotic arresting drug Colcemid. Autoradiography demonstrated the existence of two synthesizing zones, the previously described subcapsular cortex and a new zone of activity, the corticomedullary junction. The existence of these zones was confirmed quantitatively in the Colcemid-treated animals.