The aim of this investigation is threefold: (a) to determine the time of origin of neurons of the rat cranial nerve ganglia; (b) to reexamine the embryonic development of the cranial nerve ganglia in the light ofthese dating results; and (c) to attempt to relate the chronology of these peripheral events to developmental events in those nuclei of the medulla that are intimately associated with the cranial nerve ganglia. Although thymidine-radiography has been used for over 2 decades to investigate the time of origin of neurons, most of these studies dealt with central nervous struc- tures....