1. In rats, both diameter and area of the cell bodies of spinal MNs increase rapidly during the first few postnatal weeks and slowly thereafter. The total dendritic length, radial extent and arbor area of spinal MNs also increase significantly throughout the first few postnatal weeks. This development is coincident with motor development in rat, which progresses rapidly during the first two to four weeks of life. The dendritic length and radial extent of spinal MNs increase more significantly in the cervical cord than in the lumbar cord throughout the first three postnatal days, and are possibly related to the motor development, with a rostro-caudal gradient. 2. All monoaminergic neurons projecting their axons to the spinal cord are located in the brainstem. namely in the locus coeruleus, the subcoeruleus and the medulla raphe nuclei in rats. The NA neurons of the locus coeruleus begin to be detected at ED 10-13, slightly earlier than the 5HT neurons in the raphe nuclei, which are first detected at ED 13. At ED 16, the NA fibers are seen in the ventral funiculus only at the cervical level, and many NA fibers are seen in the ventral horns at all levels at ED 18. The 5HT fibers reach the caudalmost levels of spinal cord by ED 16-17, which is earlier than NA fibers; this occurs in spite of the earlier ontogeny of NA neurons in the locus coeruleus than that of 5HT neurons in the raphe nuclei. 3. The monoamine system is thought to exert a variety of modulatory effects on target neurons during both pre- and postnatal periods, and many reports support the idea that monoamine systems have a "neurotrophic effect." On the other hand, important roles of NA and 5HT in MN activity and/or motor behavior have also been reported. It is suggested, therefore, that monoaminergic systems play important roles in motor development through a two-step mechanism: during early developmental stage. monoaminergic systems mainly act as neurotrophic agents on spinal MNs, which are the final motor output neurons; thereafter, they mainly play neuromodulatory roles on MN activities.