The discharge patterns of 16 motor units in extensor digitorum longus (EDL) and soleus (SOL) muscles of freely moving adult rats, described by Hennig & Lømo (1985), were further analysed with respect to their role in grading muscle force output. The units fell into three distinct classes, termed EDL-1, EDL-2 and SOL-1, probably corresponding to type FF, FR and S units. The EDL-1 units generated only single impulses or impulse trains of short duration (phasic firing) which had high frequency and usually started with a short interspike interval (initial doublet). The EDL-2 and SOL-1 units generated single impulses and impulse trains of both short and long durations (phasic and tonic firing) without initial doublets. The frequency was high in EDL-2 and low in SOL-1 units. In EDL-2 and SOL-1 units, the mean durations of the first interspike intervals in a train decreased as the number of impulses per train increased. In EDL-1 units they did not change. Both SOL and EDL muscles were simulated through the nerve at different regular frequencies and tension-frequency (T-F) curves constructed. The EDL-2 units fired naturally most often at frequencies corresponding to the steepest part of the EDL T-F curve. The EDL-1 and SOL-1 units fired naturally most often at frequencies where the T-F curves of their respective muscles began to flatten before maximum tetanic tension was reached. Stimulus trains starting with an initial doublet produced maximum rate of tension development (optimum impulse pattern). At optimum intervals the force increased from about 20 to 85% of maximum tetanic tension when the number of stimuli was increased from 1 to 7. It is concluded that the natural firing pattern of EDL-1 units and the contractile properties of EDL muscle fibres are normally matched so that the force can develop at maximum rate to maximum levels at the start of contractions. Tension output is apparently regulated primarily through varying number of impulses per train in EDL-1 units; in SOL-1 and EDL-2 units both rate and number of impulses are important.