Motor neurons and ventral root motor fibres innervating extensor digitorum longus (EDL) muscle in rats of 0.5-3.5 months of age were studied by HRP-retrograde labelling and in vivo muscle tension recording, respectively. EDL nucleus size increased with age but motor neurons number and size did not change. Twitch and tetanus tension increased with age proportionally to muscle mass. At 0.5 months of age, but not at subsequent ages, the muscle was incompletely innervated functionally. The contribution to EDL muscle innervation came from L3, L4 (pre-eminently), and L5 ventral roots. The number of motor fibres running in L3-L5 ventral roots and innervating EDL muscle increased from 41.7 +/- 2.2 (mean +/- S.E.M.) at 0.5 months to 68.6 +/- 1.9 at 3.5 months (P < 0.001). The greatest changes in m.w., in tension and in number of alpha nerve fibres innervating the muscle occurred from 0.5 to 1.5 months of age; afterwards changes occurred, but at a slower rate. HRP labels all neurons innervating EDL muscle, while tension recordings allow the counting of alpha nerve fibres (not gamma) running in ventral roots; these differences may account for the different results obtained.