1. Nerve and muscle properties were studied in single motor units of triceps surae muscles in the cat using chronic recording techniques and intramuscular microstimulation. Recordings were made before and at intervals up to 18 months after a nerve was sectioned and sutured either to its distal stump (nerve-nerve suture) or to a muscle directly (nerve-muscle suture). Thus, each nerve and muscle served as its own control for recovery after reinnervation.2. Following a delay all muscles recovered their preoperative tension after nerve-nerve suture with a single exponential having a time constant of 1-2 months. Only half the muscles recovered their preoperative tensions after nerve-muscle sutures. Muscles which did not recover fully also had a slower time course of recovery.3. The estimated number of motor units did not increase significantly later than 2 months after nerve section and suture. Further recovery of muscle tension is due to increased unit tension, rather than increasing numbers of reinnervated motor units. Unit tension recovered completely in all muscles, but did not become enlarged, even when muscles apparently remained partially denervated.4. The latency of compound nerve potentials often recovered completely, although the amplitude of the potential remained depressed. Single motor axonal potentials recovered to control levels after reinnervation of muscle with a time constant similar to that for the recovery of motor unit tension. Therefore, two distinct populations of motor axons contribute to the compound potential: reinnervating axons whose size recovers fully, and disconnected axons which remain atrophic. Incomplete recovery of the compound potential amplitude mainly results from a failure of all axons to remake peripheral connexions.5. Thus, formation of functional nerve-muscle connexions completely reverses the effects of axotomy on nerve and muscle. Reinnervated motor units recovered their preoperative size, whether or not much of the muscle remained denervated.