We developed a novel method for measuring motoneuron regeneration using transcutaneous electrical nerve stimulation to evoke electromyograms (EMGs). Using this technique we found that functional regeneration of the motoneurons of the rat sciatic nerve was significantly slower in 9-10-month-old rats (10 mo) than in 10-12 week old rats (3 mo). All rats received crush injuries of the sciatic nerve and were analyzed for reinnervation of the muscles in the foot by the novel EMG method on days 25, 29 and 33 post-surgery. There were significantly fewer test sites reinnervated in the foot of the older group. Fifty percent of the sites in the foot were reinnervated 27.2 days post-surgery in the 3 mo group in contrast to 33.7 days post-surgery in the 10 mo group, indicative of a 24% longer recovery period in the older animals. Rats were also assessed behaviourally with a behavioural test score (BTS). The BTS results correlated with the EMG findings demonstrating that our novel method measuring rate of recovery of EMGs is behaviourally relevant. We discuss the need to use animals older than 3 months for the study of 'adult' regeneration phenomena since 3-month-old rats may be somewhat immature. We applied this age principle and the novel EMG method in our accompanying paper which shows the acceleration of nerve regeneration by weak DC electric fields.