Male Wistar rats were hindlimb-unloaded from postnatal day 4 to month 3. Some rats were also allowed to reload during 3 months of ambulation recovery. The rats were sacrificed immediately, 1, 2, and 3 months after hindlimb unloading. Numbers, cross-sectional areas (CSAs), and succinate dehydrogenase (SDH) activities of spinal motoneurons innervating soleus muscle, which were identified by the retrograde neuronal labeling by nuclear yellow, were determined. The numbers, CSAs, and SDH activities of gamma motoneurons (< 500 microm(2)) were not influenced by hindlimb unloading and reloading. The number and SDH activities of alpha motoneurons were not affected by hindlimb unloading and reloading, either. The growth-related increase of CSA of alpha motoneurons in unloaded group was slightly, but significantly, less than controls. However, the number of small-sized alpha motoneurons (600-900 microm(2)) was greater than controls, and large neurons with more than 1300 microm(2) CSA were not observed immediately after the 3-month unloading. These phenomena were gradually recovered and reached the control level at the end of 3-month ambulation. It is concluded that hindlimb unloading of neonatal rats retards the growth-related increase in the cell body size of large-sized alpha motoneurons and that such phenomenon is reversible in response to reloading during ambulation recovery.