We studied the properties of whole medial gastrocnemius (MG) and soleus muscles, and of 74 individual MG motor units in three cats with long-term complete low thoracic spinal transection (TS). All animals remained paraplegic and there was considerable atrophy of MG and soleus muscles. Motor units in the MG pool were readily classified into the same groups, types FF, F(int), FR, and S, found in normal MG, but tetanic force outputs and mean fiber areas for the corresponding muscle fiber types were decreased in all unit types as were isometric twitch contraction times. The proportion of type F(int) motor units was greater (19.0Yo) than expected from normal samples (5.8%) and this was also true of the corresponding type 2AB muscle fibers (6.9% in TS vs 3.3% in controls). In addition, FR and S units were less numerous (17.5% each) than in control muscles (23.0% and 25.2%, respectively), and histochemical examination revealed a similar decrease in the percentage of type 2A (11.6%) and 1 (16.4%) fibers compared with controls (20.0% and 30.5%, respectively). The changes found in chronic paraplegic atrophy were different in some details from those found after chronic simple limb immobilization.