Spinal motor neurons of larval frogs continue to be generated for the lateral motor columns (LMCs) even after their peak numbers have been reached and during the major period of normal neuronal loss. This study elucidates the extent to which relatively late generated motor neurons survive to the end of larval development. Bromodeoxyuridine was injected into Rana pipiens tadpoles at mid-larval stages twice a day for 5 days in order to label LMC neurons arising during the period of greatest neuron loss. Counts of labeled neurons visualized by immunocytochemical methods revealed that substantial numbers of late generated motor neurons survive to the end of larval development and may be retained in greater proportion than early generated motor neurons (prior to the onset of normally occurring neuron loss). Tadpoles that had been exposed to exogenous thyroxine so as to increase proliferative activity and thereby label more motor neurons during the injection period, exhibited an increase in labeled LMC cells although the total number of neurons remained comparable to controls. Protracted neurogenesis concurrent with cell loss during the later phases of development is a major determinant of the final population of the LMC.