This study seeks to identify the mechanisms which motoneurones use to control the contractile force and speed of skeletal muscles. We have stimulated directly slow soleus (SOL) and fast extensor digitorum longus (EDL) muscles of adult rats intermittently at 100 Hz for 1-9 months. The muscles were either chronically denervated, denervated and reinnervated, or normally innervated. The stimulation started either immediately, or more commonly, after 1-9 months of denervation. Stimulation starting several months after denervation increased the mean maximum tetanic tension 37 times in SOL and eight times in EDL. These values represented 40 and 12% of the increases obtained by reinnervation after comparable periods of time. In denervated SOL and EDL muscles stimulated directly for more than 2 months, the mean isometric twitch contraction times were 13 and 12.7 ms, as in normal EDL muscles (13 ms). In innervated SOL muscles stimulated directly for 1-4 months, the mean twitch contraction times were 23.6 ms (normally innervated) and 19.2 ms (reinnervated), which were considerably shorter than in normal control SOL muscles (39.2 ms). Single motor unit recordings revealed that the natural (background) nerve impulse activity was essentially unaffected by the stimulation. Twitch contraction time and percentage of type II fibres in SOL muscles were related. The fastest muscles (denervated and stimulated) consisted of 100% type II fibres (with one exception), the second fastest (reinnervated and stimulated) of 70-50%, the third fastest (normally innervated and stimulated) of 45-0%, the second slowest (reinnervated) of 15-0%, and the slowest muscles (innervated controls) of 5-0% type II fibres.