The ageing neuromuscular system is thought to undergo a continual process of reorganization as motoneurones are lost and surviving motor nerves reinnervate neighbouring denervated muscle fibres. However, the extent to which collateral reinnervation is able to compensate for neural deficits in the ageing individual is unknown. The ability of the senscent motoneurone to increase the size of its peripheral field was therefore investigated following transection of the right L 5 ventral root in male Sprague Dawley rats aged 775 ± 50 days. This procedure resulted in an extenssive partial denervation of the right medial gastroscemius muscle. After a recovery period of between 28 and 31 days the isometric contractile properties of surviving motor units were compared to control units from both the contralateral muscle and a group of unoperated control animals aged 791 ± 39 days. Motor unit force was found to be unchanged after partial denervation and the absence of any alteration in motor unit size was confirmed by histological analysis. However, the time course of the isometric twitch was significantly longer for both fast and slow motor unit types and the conduction velocity of motoneurones innervating fast unitswas decreased following partial denervation. These results demonstrate that senescent motor nerves are unable to substantially increase the size of their peripheral fields by extensive collateral reinnervation.