An autoradiographic determination of the time of origin of the lateral motor columns (LMC) of the chick embryo has been made. The first motor neurons of the brachial LMC are born at stage 15; the earliest birthdates of lumbar LMC neurons are at stage 17. At least 95% of the motor neurons of both brachial and lumbar columns are produced by stage 23 (4 days). The remaining 5% of the motor neurons are produced during the next two days. A clear rostrocaudal gradient of motor neuron production is seen beoth between the brachial and lumbar LMCs and within the LMCs themselves. The LMCs are assembled in a mediolateral sequence: the early-born motor neurons settle medially, the later-born motor neurons settle more laterally. Observations were made of other large early-born neurons which remain permanently in the dorsal gray of the spinal cord.