Effects of non-weight-bearing (NWB) on morphological and metabolic properties of motoneurons innervating the soleus muscle, a postural muscle, were studied in young growing rats. The soma size and the activity of succinate dehydrogenase (SDH), a mitochondrial enzyme, were examined after hindlimb suspension. The hindlimb suspension began at 21 day of age and lasted for 3 week. The results were compared with 21- and 42-day-old controls. The soleus motoneurons were identified by using a fluorescent retrograde tracer, nuclear yellow. The histochemical SDH activity of soleus motoneurons was not modified during the postnatal growth and after the NWB condition. However, postnatal increases in the soma size and the total SDH activity (SDH activity x soma size) were arrested by the NWB condition. The results indicate that the weight support activity imposed on soleus motor units would be important to postnatal increases in the soma size and the total SDH activity (i.e. mitochondria) of soleus motoneurons. (C) 2000 Elsevier Science Ireland Ltd. Ail rights reserved.