Motoneuron cell death in the lumbar lateral motor column of the mouse embryo and neonate was examined to determine the timing and position of cell death with respect to events occurring in the limb. Counts of motoneurons in histological sections of the entire lumbar lateral motor column were made in mice ranging in age from 12 1/2 days of embryonic development to 20 days of neonatal life. Between 13 and 18 days of embryonic development, 67% of the motoneurons initially present in the motor column die, or approximately 3350 out of 5000 cells. Peak motoneuron cell death occurs at 14 days. No cell death occurs during the first neonatal weeks when polyneuronal muscle fiber innervation is lost. Counts of the total number of cells present at 18 days were similar to those previously reported for adult mice, suggesting that all motoneuron cell death has occurred by the end of prenatal development. In a few 13- to 14-day mouse embryos, the hindlimbs were totally filled with horseradish peroxidase (HRP) to define the boundaries and size of the lateral motor population which projects to the limb at early stages. Counts of HRP labeled motoneurons in selected lumbar cord segments were close to the total number of lateral motoneurons in the same segments. As the HRP injections were made prior to or at the onset of cell death, these observations indicate that many cells which die have sent axons into the limb.